if you have married your daughter's father, you can change it at the register office where your daughter's birth was registered for free, just need to pay for new certificate �3.50
if you married somebody else, there are two types of deed poll.
i think that one is around �20 and its just one that the solicitor makes up himself, if you get this one, get lots of duplicates as no record is kept and if it is lost, you will have to do the whole thing again.
there is another type of deed poll that is a bit more money, however the solicitor will keep a permanent record so if it is lost, or your daughter wants another copy when she is older, she can just go to the solicitor and request one
out of the two, this is the better one