In reply to minimike:
Largely depends on circumstances of both you and the seller what you can get away with. Essentially the more screwed they are and the more flexible you are the lower it goes.
Best case scenario - you're a cash buyer, they've already bought their next house and are currently paying out for a bridging loan at stupid interest rates, and their house is a wreck and no-one else will look at it.
Under that situation I'd initially offer 30% under asking.
Given you're FTB with no chain you're in a pretty good situation. Only you will know their situation.
Bear in mind people (the seller and the estate agent) will *expect* your first offer to be both low and a lot less than you can actually afford - so make sure it is. They will also, for the same reason, expect you to expect the first offer to be rejected.
I wouldn't go more than 90% of the asking in any case for a first offer. In this case as you are FTB I'd lower that to 85%.
[quick maths] I'd probably start at 145k or 150k. Obviously this will be rejected (well, you never know!) and I'd then go to 155k and ask for a counter-offer.
Say you then get 165k as counter and you then offer to split the difference and get it for 160k.
FWIW, on our house we went in with an extremely cheeky 20% discount from asking and the sellers said "yes", as they were in bridging land and the house was knackered.