You'll find all the information you need here:

http://www.seat61.com/Belgium.htm#London%20to%20Brussels%20by%20Eurostar

SM

Thanks very much for the link - just what I needed

We did this last year and it was very straightforward - I think they even had info about changing stations in the Eurostar magazine on board. My review is here if you are interested - http://www.holidaytruths.co.uk/forum/bruges-t79157-20.html

I think it might be brussels midi that you change at

Brussels midi is indeed the station you arrive at.

nmbs.be is the official belgian rail website if you need help

seems like most people go to Bruges, but I'd recommend going to Ghent (Gent in flemish)
it's similar to Bruges but not as touristy and less likely to get ripped off... stay at the Marriotts.. one of my favourite hotels...