Practice takeover

I am working as a locum at a practice. The solo GP's spouse is the practice manager and they both plan to retire soon. If I can take over the practice, they will let me buy the premises (purpose-built in the 1980s).They want £700,000 for the building plus £200,000 for the fixtures and fittings. The notional rent is currently £23,450 a year (grossly below what a surgery worth £700,000 should attract) and is due for review in 2010. I think the premises are only worth £300,000, while the couple claims they were valued at £800,000 in 2007. However, they have no documentation to support this. They simply say they reduced the price because of the credit crunch. I suggested the GP takes me into partnership and leases the surgery to me but this was unacceptable because it may attract the taxman's attention. How I can convince them to sell at a reasonable price?