No part number required, the same wheel bearing was fitted to every 25 / ZR / 200, and I also believe it was fitted to the 400.
You should be able to just walk into a unipart and request a Rover 25 front wheel bearing.
There are also plenty for sale on ebay, here's one for example:
http://cgi.ebay.co.uk/ROVER-MG-ZR-25...item19b8d3e79f