Thanks for responses all.
DD's school is actually quite laid back compared to some of the other grammars in the area and they don't accelerate for anything other than maths and some for science. Those that do maths in year 10 (about half the year) no longer do AS1 in year 11 but something called "additional maths". The school used to do AS1 in year 11, but changed its policy once the A* was introduced at Alevel. It felt that the girls would have more opportunity to get an A* if they waited until year 12 to take the exam - hence the introduction of this "additional maths" in year 11.
For those girls who want to take maths to Alevel, I think the acceleration is probably a good idea. They get the opportunity to consolidate their maths in year 11 and then progress to the advanced stuff in year 12. For those, like my DD, who can't bear it, the thought of doing any more than she has to is horrendous.
To be fair on the school, the letter says that it's not compulsory to be in the accelerated group, so I think I shall just bite the bullet, say no and put a little explanation as to why. Let's just hope they understand.......