Cheshire Cat
The most curious thing..
I have never insisted on anyone taking time off at a particular time (as there has never been a need to do so), but I have refused leave requests in the past from staff for various work-load related reasons. However I think companies can regulate when you can and cannot take leave (although it is always better if individual requests are mutually agreed with the manager and staff in advance), especially where shift work is involved. Speak to HR because in my experience line managers often don't actually know the detailed ins and outs of their company policy. The contract itself probably won't help as it may not go into enough details about your particular issues, which is why you need to confirm the process with HR (that is what they are there for) assuming that HR actually understand the process - which in itself may not be guaranteed.