Hi there,
None of can be certain about direction of market, but I think with current sentiment in the papers etc, and IIB/ IAVI all predicting at least 5% falls in prices in 2008, I would offer the following advice
Forget what the owner thinks his property is worth. The fact he wants to sell to you as sitting tenant and not take his chances with the market, makes me wonder. Did he initiate contact on sale? If he doesn't play ball let him sell see how much interest there is out in the market. I think he will know market is dead at moment?
He has priced house, above 2 other houses in area, knowing you will offer below his asking price. Don't be embarrassed to offer well below lowest price in the estate. Remember te mortgage repayments on this will take at least 20 years. Every thousand you can save now, will reduce the term of your mortgage.
Have you used Thepropertypin.com; treesdontgrowtothesky.com and other property sites to see what is going on in your area. These might give you a good idea of the pricing
Is the landlord under pressure to sell? Is he involvved in property development, buying selling stocks. Either of these and he might be under more pressure to sell than you think
If you love the house, and it is unique for your requirements, and is worth the price, and more improtantly you can afford the price, well go for it.
Offer 20% below the current market value of the property (not his valuation of his own asset!!) I would offer 25% less, as no EA fees involved, but 20% is worth a try. Don't blink. Leave offer on the table for 3 days only. If he does not agree to it by then if he returns to you reduce that offer by 1% per week.