I'm in the same boat!!
You need to find a 24 hour airfield (Southend, Bournemouth, Blackpool were a few I looked at) - not sure about Coventry? However, if this isn't your local field, you will be wasting time and money by flying to the 24 hour field if you are using your existing school. However, you may find another school just to do the night rating. Try google for schools.
The other option (which is my course of action due to the nature of night flying in helicopters) is to get your existing to obtain a dispensation from the CAA i.e. you do the training and CPL skills test and undertake the night hours at a later date.
This is not an uncommon scenario (especially in the rotary world); what does your instructor suggest?
Cheers
Whirls