1. A hand held apparatus for creating an aerosolised mist of particles, the apparatus comprising a high pressure vessel, a spray nozzle and a valve, said valve comprises a body fixed to the high pressure vessel and a metering element wherein the valve body having an inlet in communication with the high pressure vessel, and an outlet in communication with the spray nozzle, the metering member including a metering chamber mounted in the body which is adapted to move between a first position in which it is in communication with the inlet and a second position in which it is in communication with the outlet, whereby the movement of the metering member between the first and the second positions is performed along the axis, perpendicular to the pressure action at the inlet. 2. An apparatus according to claim 1 wherein the valve body is provided with a bore, the metering member is defined by a shaft, the metering chamber is a cut-away section of the shaft and the shaft is seated in the bore. 3. An apparatus according to claim 2 wherein the bore has both ends open to the atmosphere. 4. An apparatus according to claim 2 or 3 wherein seals are disposed between the body and the shaft to prevent the escape of liquified gas. 5. An apparatus according to claim 4 wherein seals are arranged such that the inlet is at all times isolated from the outlet irrespective of the position of the shaft. 6. An apparatus according to claim 5 comprising 2 pairs of seals such that when the metering chamber is in the first position a first pair of seals keeps the metering chamber in communication with the inlet and isolated from theoutlet and when the metering chamber is in the second position a second pair of seals keeps the metering chamber in communication with the outlet and isolated from the inlet. 7. An apparatus according to any one of claims 4 to 6 wherein the seals comprise PTFE lip seals with an integral metal spring.  8. An apparatus according to any one of claims 2 to 7 wherein the cut-away section is is the form of a chamfered groove. 9. An apparatus according to any one of claims 1 to 8 wherein the high pressure vessel is provided with a rupturable closure member and the valve body at the valve inlet is provided with means to rupture the rupturable closure member such that when the high pressure vessel is engaged with the valve body at the valve inlet the rupturable closure member is ruptured. 10. An apparatus according to any one of claims 2 or 3 wherein the valve inlet is provided with a pin valve comprising a pin biased against a seat and the shaft includes a ramped surface such that in the first position the pin of the pin valve is displaced from its seat to allow communication between the highpressure vessel and the metering chamber and in the second position the pin of the pin valve is seated to prevent communication between the high pressure vessel and the metering chamber and wherein the displacement of the pin from its seat against the bias is caused by urging of the ramped surface against the pin upon movement of the shaft between the first and second positions. 11. An apparatus according to claim 10 wherein the valve body is provided with a second outlet and wherein the shaft includes a second ramped surface which cooperates with the first ramped surface such that the shaft may move between the first position in which the metering chamber is in communication with the inlet and the second position in which the metering chamber is in communication with one of the two outlets on moving the shaft laterally in either direction within the bore. 12. An apparatus according to claim 10 or 11 further provided with seals which are arranged such that the inlet is at all times isolated from the outlet or outlets irrespective of the position of the shaft.  13. An apparatus according to claim 12 wherein the seals are manufactured of PTFE. 14. An apparatus according to any one of claims 10 to 13 wherein the high pressure vessel is provided with an integral valve assembly comprising a member biased against a seat such that when the high pressure vessel is engaged with the metering valve inlet the vessel valve member lies in close proximity to the pin of the metering valve pin valve such that on actuation of the metering valve the pin of the metering valve pin valve is pushed off its seat and abuts against the vessel valve member which is in turn pushed off its seat thereby bringing the inlet into communication with the interior of the vessel. 15. A method for creating an aerosolised mist of particles which comprises forming a solution or suspension of the substance to be aerosolised in a high pressure liquefied gas which is in a sub-critical state and rapidly expanding the solution or suspension at the spray nozzle of an apparatus according to any one of claims 1 to 15. 16. A method accrding to claim 15 wherein the liquefied gas comprises carbon dioxide, nitrous oxide or sulphur hexafluoride. 17. A method according to claim 15 or 16 wherein the substance to be aerosolised is a pharmaceutical. 18. A method according to claim 17 wherein the pharmaceutical is salbutamol, beclomethasone dipropionate, salmeterol fluticasone propionate or a pharmaceutically acceptable salt of any one thereof.