Hi guys, very many thanks for your views on this issue.
Aragorn, I have read that the best way to remove leaked coolant is to hire a wet and dry vacuum, like the Rug Doctor that is widely available. A couple of people posted on other sites and suggested putting some small amount of clean water onto the carpet and then use the wet and dry vacuum to hoover it out, along with the coolant. The suggestion was to do this a few times to get as much coolant out as possible. Then put in a dehumidifier.
That sounds like a fairly sensible solution but again fairly long term. The very immediate issue is stopping the leak, and Richards' point is probably the best one - to put in a by-pass link tp prevent coolant going through the heater matrix. The only thing that I have against this is that I will be driving a cold, cold motor if I have to use it. But, obviously, there has to be some sort of compromise, there's no point in constantly complaining that there's a leak and then doing nothing about it !!