Thanks for all your replies.
Nice!
My feeling is that they are trying it on. I am almost certain that they can't seize my car as it is not owned by me due to being part of the motability scheme. In respect of my other goods, I still don't think they can seize anything as they do not have a court warrant.
Anyway, i've written to them making these points and explaining that I will continue to make the payments under the agreed installment plan. We'll see what they say.