OK here's the scenario... (apologies in advance for all the numbers and complicated house nomenclature, I found I couldn't make sense when I tried to put it down in abstract terms!)
Our house 'House A' has been on sale since January. In a very slow, falling market.
Our initial asking price £315K. IN May we accepted an offer of £284K from our buyers who had an offer on their House B for £134K.
Found house we really like 'House C' - was on at 'offers over £450K' but we eventually agreed to pay £425K for it.
Chain collapsed (House B buyers pulled out) in June so all back to square 1.
We lowered asking price of House A to £295K but very little interest over the summer. House C also reduced to offers around £435K.
Our original buyers eventually found new buyers for house B at £125K and subsequently made an offer on our house A at £274K which we accepted (hence we had absorbed all of the shortfall). In spite of an attempt to bargain with vendors of house C the best agreement we could come to on theirs was £423K. My husband did the negotiation and I think we are overpaying at that figure but the house really suits us so was prepared to live with it.
Now the chain has fallen through again (sigh!) so houses A, B and C are all again 'for sale'. However there does seem some renewed interest in our house so we might still get a sale.
Today we heard via our EA that another EA who lives on the same street as house C believes it is worth £400K 'at best'.
Question is, if we do get a new offer for ours, is it possible/ethical to start negotiation AGAIN on house C? With the hope that we'd get it for something near it's actual market value. I do worry that (through no fault of our own) they already see us as 'flaky' buyers and they may just walk away. But I am equally sure that they are not very likely to get anywhere near asking from anyone else and that in 6 months time their house will be worth even less. I don't want to be grossly overpaying for the house no matter how much I like it. Complicating factor is that husband is completely in love with it and will be totally gutted to lose it. Hence our rolling over immediately on all negotiations to date...'how much would you like us to pay for your lovely house that we love? Oh, ok then are you sure that's enough?' 
Thanks to anyone for taking the time to read this far, Would appreciate any advice.