Thanks so much for your help - at least we can see that there is something we can do to try and fix the problem. The house is only 3 years old, with no hard water. The screaming only happens when we turn on the hot water at the shower - oher sinks are okay.
My skill level is not that high in this area so I will get a plumber to check it out - the main thing I was unsure of was that it was actually a problem which could be fixed. Some people have said to me that there is nothing you can do and you just have to live with it - a bit savage that one though!