Why won't Siri recognise my home address?
OK I've found out the answer myself.

It turns out that although the Maps app can geocode (find the location of) my address, Siri can't. It must be using a different geocoder to the Apple Maps app. 

After much faffing, I found the easiest way to fix this is to drop a marker on my house using the Apple Maps app and then add this to my own contact record as "home". This fixes the problem and allows Siri to know where home is. However it then means my address is incomplete if I use it for other purposes or in other apps). The only way round this seems to be add it back and tag it as "address" (NOT "home") as a second copy of the same address. Really messy.

Clearly this is a bug in Siri which will hopefully be fixed one day (I won't hold my breath). I don't recall having this problem in iOS 5, so I think this was introduced in iOS 6.