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As others have said, pushing down the price at the moment could be a struggle.
I think it depends on the circumstances. We got £10,000 knocked off the price after the bank valuer said the flat was worth £20,000 less than we had offered. I think their valuation was plucked out of thin air as there hadn't been many sales on the estate recently. The estate agent had a long meeting with the vendors and explained our situation to them. We would have walked away if we hadn't got a reduction.
We have just completed the purchase today after four months of (unrelated) legal complications.
pjay-
I think the bank valuer is a different issue though. Effectively it's limiting the mortgage and the amount that people are going to pay.
Trying to knock off £10k because the windows are knackered, boiler will need replacing soon, etc is a bit more difficult. Although if you're far enough along it may happen rather than starting it all again.
aggi
As others have said, pushing down the price at the moment could be a struggle.
On the other hand you don't want to buy somewhere and discover it immediately needs £40k of work to be made safe.
For the price you may as well.
There is the in-between option of a homebuyer's report. Gives some info but there will also be a reasonable amount of "this could potentially be an issue, have an expert look at it". Depends if you're happy with that.