The problem with a lot of the cheapo HIP suppliers is that they might not do all the searches/surveys correctly and you end up with certificates and HIP pack that isn't really right.
The option of doing it yourself might sometimes look good but by the time you;ve paid for one off surveys the cost does add up and is sometimes a lot more than what an agent might get as he can get trade discounts..
As said £250 looks an OK price, I have seen a lot more and a lot less, just don't though go on a price and expect quality for a more expensive price.
You could go look at a few houses, pretend to be a buyer, gets lots of free coffee and biscuits and just mention that you've gotta sell your own house and ask who did thier HIP (will probably be printed on the HIP somewhere anyway
)