There are a finite number of reasons that will justify the refusal of a flexible working request these are:
– the burden of additional costs;
– detrimental effect on ability to meet customer demand;
– inability to reorganise work among existing staff;
– inability to recruit additional staff;
– detrimental impact on quality;
– detrimental impact on performance;
– insufficiency of work during the periods the employee proposes to work; or
– planned structural changes.
You should not refuse a request just on the basis of a reasons above though, you will need to demonstrate why any of the above apply.