1. A macromolecular compound having attached to said compound one or more molecules (neuraminidase binders) which bind to the active site of influenza virus neuraminidase but which are not cleaved by the neuraminidase. 2. A macromolecular compound according to Claim 1, in which the neuraminidase binder is attached to the molecule via a spacer or linker group so that the neuraminidase binder is no sterically hindered by the backbone of the macromolecule. 3. A macromolecular compound according to Claim 1 or Claim 2, in which the neuraminidase binder has an IC50 of 10<-6>M or less. 4. A macromolecular compound according to any one of Claims 1 to 3, in which the neuraminidase binder is a sialic acid derivative. 5. A macromolecular compound according to Claim 4, in which the sialic acid compound is compound (I)  compound (I) GG167   functionalised via the 7-position of the sialic acid structure. 6. A macromolecular compound according to any one of Claims 1 to 5 of formula (II): (X-Y)n-M-(Z)m       (II) wherein X is a neuraminidase-binding 2,3-dehydro-sialic acid derivative (2)   which is linked at the 7-position via a spacer group Y to a macromolecule M, and Z is an optional extra substituent on the macromolecule. in which the spacer Y connects to the W group, and wherein R represents an azido group, an unsubstituted or substituted guanidino group, or an unsubstituted or substituted amino group; R<2> represents COCH3, COCF3, SO2CH3 or SO2CF3; W represents O(C=O)NH, O(C=S)NH, NH(C=O)NH or NH(C=S)NH and is attached through the NH to group Y; m is an integer between 0 and 1000; and n is an integer between 1 and 1,000, the spacer group Y is an optionally substituted chain of up to 1000 atoms chosen from carbon, nitrogen, oxygen and sulphur; and the macromolecule M is a synthetic or natural polymer, protein, antibody or enzyme of molecular weight from 10<4> up to 10<7>, linked covalently or non-covalently to the spacer group Y. 7. A macromolecular compound according to any one of Claims 1 to 3, of formula (IIA) (X'-Y)n-M-(Z)m              (IIA) wherein X' is a neuraminidase-binding cyclohexenyl derivative of formula (2a),   which is linked through the ether side-arm, R, R<2>, Y, n and m are as defined in Cla im 6 , and R<1> and W<1> are lipophilic C1-C12 alkyl or alkylene groups which are optionally substituted by one or more halogen atoms or alkoxy, haloalkoxy or optionally substituted aryl groups. 8. A macromolecular compound according to Claim 6 or Claim 7, in which Z is a group which binds hemagglutinin. 9. A macromolecular compound according to Claim 6 or Claim 7, in which Z is a group which can act as a detectable label. 10. A macromolecular compound according to Claim 7, in which Z is biotin or a fluorescent molecule. 11. A macromolecular compound according to Claim 6 or Claim 7, in which Z is an enzyme suitable for use in a detection assay. 12. A macromolecular compound according to Claim 6 or Claim 7, in which T is a group with a terminal functionality that is suitable for binding the macromolecule to a surface. 13. A macromolecular compound according to Claim 12, in which Z is NH2, SH, CO2H, CHO or CH=CH2. 14. A macromolecular compound according to any one of Claims 6 to 13, in which the spacer group Y is selected from the group consisting of aminoalkyl groups, (poly)amino <DP=4acids, linear peptides, oligosaccharides and polysaccharides, polyethylene glycol units, and aminodialkylureas, any of which may be used alone or in combination. 15. A macromolecular compound according to Claim 14, in which Y has a terminal amino group. 16. A macromolecular compound according to any one of Claims 6 to 15, in which the macromolecule M is selected from the group consisting of proteins, enzymes, antibodies, water-soluble synthetic polymers, polysaccharides and polyaminoacids. 17. A macromolecular compound according to any one of Claims 6 to 15, in which the macromolecule M is selected from the group consisting of serum albumin (BSA), horseradish peroxidase (HRP), avidin, streptavidin or neutravidin, and immunoglobulins. 18. A macromolecular compound according to any one of Claims 6 to 15, in which the macromolecule M is selected from the group consisting of polysaccharides, polyacrylamides, polyethylene glycols, polyureas, polyacids, polyesters, polyamides and N-(2-hydroxy-propyl)methacrylamide (HMPA), said macromolecule being pharmaceutically acceptable. 19. A macromolecular compound according to any one of Claims 6 to 18, in which R is a guanidino or amino group substituted with a methyl, ethyl, allyl, amino, cyano or nitro group. 20. A macromolecular compound according to Claim 6, in which X is a compound of formula (2), in which R is guanidine, R<2> is acetyl, W is the group O(=CO)NH and the spacer Y is a chain made up of between 6 and 60 carbon, nitrogen and oxygen atoms. 21. A method of detection of influenza virus, comprising the step of exposing a sample suspected to include said virus to a compound according to any one of Claims 1 to 20 which is able to bind specifically to the active site of influenza virus neuraminidase. 22. A method according to Claim 21, in which the compound is attached to a surface via covalent binding or non-specific binding, and in which the spacer group Y is sufficiently long that the neuraminidase binding units X are exposed on the surface of tree macromolecule M and accessible to a virus particle. 23. A method according to Claim 21 or Claim 22, which is a selective capture assay, a selective detection assay, or a combined selective capture-selective detection assay. 24. A pharmaceutical composition for treatment of influenza A or influenza B comprising a compound of the invention, preferably a compound according to any one of Claims 1 to 7, 13, 14 and 18 to 20, or a pharmaceutically-acceptable derivative thereof, together with a pharmaceutically-acceptable carrier, in which the compound has an ID50 for neuraminidase of less than 5 μg/ml. 25. A composition according to Claim 24, in which the compound is a compound according to Claim 6 or Claim 7. 26. A composition according to Claim 24 or Claim 25, further comprising one or more further therapeutically active agents. 27. A composition according to Claim 26, in which the further therapeutically active agent is an anti-viral agent. 28. A method of treatment of influenza infection in a mammal, including man, comprising the step of administering an effective amount of a compound of the invention, preferably a compound according to any one of Claims 1 to 7, 13, 14 and 18 to 20, or a pharmaceutically acceptable derivative thereof, to a mammal in need of such treatment, in which the compound has an ID50 for neuraminidase of less than 5 μg/ml. 29. Use of a compound according to any one of Claims 1 to 7, 13, 14 and 18 to 20 for the manufacture of a medicament for the treatment of an influenza viral infection, in which the compound has an ID50 for neuraminidase of less than 5 μg/ml.