Go here:
http://www.statistics.gov.uk/census2001/cn_105.aspAbout five lines from the bottom, click on the CSV link.
It downloads a file that extracts itself into another set of files. One of them is an explanation of the headings in the other files, which makes the nonsense mean something. You'll need Excel or Open Office to read them.
From that lot, you can check by postcode sector how many semis, detached, flats etc there are.
All data from 2001 but that should be OK for most areas.
Hope that helps,
Vin