Pay at the moment is not fantastic but it keeps you in the aviation environment, if your doing your cpl's it will help a fair bit with getting a job. Every FISO i have worked with who had a cpl/atpl have got flying work from being in the tower.
Pay during training is becoming a rare thing now. Some small airfields (certainly the one i work at) require the commitment of working for free during your training. Pretty crap deal to be honest but if your confident of being able to pass then you get a job out of it in the end.
The other thing is that for a first validation it will normall take around 70-80 hours of training. 40 is the minimum but not the norm unless you are experienced.
Definately do the writtens first under your own steam, i believe they are now held 4 times a year at Gatwick, not sure on the cost but you will only need to take 2 of the 4 as you hold a PPL.
DBB