Have you seen that the Land Register of England and Wales is now online? For £2 a pop you can download the register entry for any property.
Interestingly it looks like there's an entry for a property at SW1A 1AA but not SW1A 2AA. (Put those postcodes into the system to see what I'm talking about.)
Even better for nosey neighbours, at least one company - Nethouseprices.com - has bought a copy of the database and is providing a free service whereby you can find the value of any property transaction completed since 2000. (So that's how much my ex-neighbour made when he sold up after six months...)
The BBC has an article with links to further sites.
Have fun!
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