Based on the above then, I think 440-450 is a serious offer to start the conversation: can you post some of the other houses so we can compare?
Btw - I expect vendors really are looking at poss 460: the house is v done so they WILL have bought with intention to do up and sell
Look hard online,you might find the listing from a few years ago to find out what they’ve actually done (you want see structural work like a reroof etc which costs the real money)
Is this house what you want? Are others coming on, and are you fully proceedable?
Also, if you offer give a deadline so they can’t just use your offer on table to drive up others: once your deadline has gone, retract