As it's not going to sealed bids on a closing date, then you have nothing to lose by putting in an offer.
if it is not accepted then of course you can put in a higher offer.
you think, after the work is done, it could be worth £295, you think the work needing doing would be around £40/£50,000 - I would allow at least 10%-20% more so aim for £60,000 to do the work
and of course the property may not be worth £295 afterwards maybe 5% less ? so it could be around £280,000 ish.
£280,000 - £60,000 = £220,000
I would offer around £220/£225 - which is 10% under the offers over price.
But then like someone else I wouldn't expect the vendor to take the hit of paying for the work needed as they are not going to benefit from it.
I would be offering a % above the offers over asking price, and it would be an odd percentage not the typical 5% or 10% but something weird like 8.3% but that would be £20,750 which puts the offer at £270,750 !
which is way above the house valuation !!!
and they will bite your hands off !
So it comes down to, what can you afford - are you a cash buyer or is there a mortgage.
and of course have a look around and see if you can find sold prices in the local area for similar properties.