I have my mobile phone back. It doesn't seem to be charging up any better than before I took it in to be fixed, dropped the loan phone in water and was told I would be charged £50 for not returning it in good condition.
It's been a learning experience and here are some of the lessons:
1) Don't drop a phone in water, it damages the phone.
2) If you do, don't try and turn it on, this may well be what causes the damage.
3) Open the phone, take out the battery and SIM card and put everything else in a bowl of dry rice. It didn't help me at all, but apparently it can help. At least I felt I was doing something.
4) It is worth complaining. A very nice lady from T-Mobile called me in response to my letter explaining that I thought charging me £50 to replace a phone I could buy for £25 on-line was unreasonable and extortionate. She said she would credit my account with £25. And she thanked me for writing. You're welcome!
5) I am far more attached to my phone than I realised. Very few people call me, or text me. In fact, it is mostly my best friend. I have set the phone to ring a different tone if it's her calling, but as it nearly always it, I don't really need it. But I like it. In fact, I really like my phone. I'm glad I've got it back!