Dear all,
Thanks for all your advice. I thought I would conclude the story, while at the same time offering some advice.
I made it to the Apple Store today having booked an appointment. The genius took it away, took it apart and diagnosed the problem pretty quickly. The water had got through the earpiece, necessitating a new screen. The logic board was tested and was fine. No water had got into the headphone socket, the speakers or the charger. All good. Cost for replacement screen was £79. For the record, the cost for a new logic board (essentially a new phone) would have been £129 through Apple
So why am I writing all this? Well, it was simply to encourage anyone wth the same problem to go to the Apple Store for the diagnosis and then repair. Admittedly, the cost of the screen was higher than the price other firms were charging for the part, but there was no guarantee that the repair that they made would work. Many firms were asking for a fee just to look at the phone.
I expect a lot of people will write in and say that I am doing a lot of firms a disservice. I probably am. They no doubt will charge less for the repair. However, my experience of the Apple Store was that the problem was solved quickly with plenty of help and advice on the way. The genius even recommended a waterproof case that was not sold in the Apple Store! If the problem had been deeper than the screen (eg a new coil), I think other firms would have added more and more extras before finally solving the problem. I would be paying for their 'guesses'. One firm for example said if it was the logic board, a new phone (reconditioned) would cost £180.
Anyway a happy ending. Thanks again for all your help.