For example, searching for "36 High Street Clapham London SW4 7UR" using the geocoder gives a significantly different location from the main map search, rendering the geocoding service useless, for my purposes at least.
Anyone know why this is and if there's a workaround?
Well for one thing there's nothing that says the map search uses the geocoder service. Clearly, it's related, but may not be the same thing.
Also, the main map search is influenced by the current viewport of the map pane. If you're zoomed in one one country or location, the search will be biased by that. if you use the geocoder, you can supply various optional parameters including viewport bias.
One thing to watch out for in UK geocoding is that the Royal Mail owns the copyright to UK postcode geographic information, and has refused to sell Google a licence to redistribute that information. Google has a licence hat allows it to use the information for geocoding on its own websites, but not for other people's websites. That doesn't explain that particular address, but it does affect the geocoding of many UK locations.
There was a bug in the API geocoder (maps.google.com geocoding was not affected) a while ago that caused it to get wrong information about the existence of certain house numbers in a street. It's possible that this might be happening again. Note that "34 Clapham High Street, London" geocodes correctly, but the API thinks that 36 doesn't exist, and just returns a location with accuracy=6 (street) instead of accuracy=8 (address). This might possibly be happening again. If you think that's what's happening, find several convincing examples and post them to the discussion group.