We made offers on 5 different houses before we bought this one. What matters is what is fair, what you can afford isn't really a concern of the sellers. We only got £4,500 off the asking price of this one, but it was on the market for around £20,000-£25,000 less than others on the street had sold for in the last 18 months, yet had a big extension and the seller really wanted to shift it before she got repossessed. Go on Zoopla/Mouseprices etc, and put in the street name, and see what houses on that street have sold for recently, I know zoopla will even give you an estimate of what it thinks the house is currently worth. This is only a guide though, as the house down the road which sold for £15,000 less than this one is on for might have needed a new kitchen/bathroom/decorating etc, whereas this one might be completely renovated.
The other thing which may make a difference is how long the house has been on the market for, and if the seller is keen to shift it. If you use firefox download the propertybee addon, it will track all changes to house prices on rightmove so you can easily see if it was listed 3 years ago and has never dropped (seller won't shift) or has been on 3 years and dropped 10k every 3 months (seller really wants to sell). If it's am empty house (owner died/went into care/moved away for a job) or a separated couple they may well sell quicker than a family who have no reason to move quickly and can hold out for the right price.
Of course with house buying people can be quite attached to their house and are sometimes unrealistic. For example in January 2010 we saw a house on the market at £145,000. It needed a LOT of work, had been empty a while, damp, no central heating, new kitchen and bathroom needed, structural changes needed to make it a modern family home, we looked into prices in the village and made an initial offer of £118,000, which was refused. We went up to £121,000 in the end, also refused. I recently drove past and saw it had sold, popped on Zoopla to see that they eventually got £115,000 for it in August 2011. So we were about right on what it was worth as far as I'm concerned, and the owners missed out on over a year of having that money in their pocket while the house sat empty and got progressively damper.
Best of luck in house hunting, it's a really exciting but also stressful time. x
