Dependant upon how often you intend to fly pay them in £500-1000 chunks, that way if anything does happen you wont get too badly burnt. Certainly do not pay them £5k!!!!
If they want £5k up front and wont accept anything smaller then I would question their motives and look elsewhere.
Ask about their refund policies if you want to get your balance returned for whatever reason, some flying schools have been known to charge student for refunds and in some cases run off with their entire credit balance!
J.