I was invited along to the grand opening of the Champagne bar in the new Eurostar terminal at St. Pancras the other evening and a very nice lady from Eurostar told me that if you travel to Belgium i.e. Brussels and change for another destination in Belgium, then that part of the journey is covered by your Eurostar ticket.

I only can confirm that a eurostarticket from London to Brussels give you the right to use Brussels (Gare Midi) as transit for any other station in Belgium.

Gerrit

Thanks for the tip Sam might give it a go next year