I'd have thought the obvious answer would be to serve a SARN on them...will cost you a tenner but will:(posted 8227 days ago)1. Cause them aggro 2. Cost them more than a tenner (think of all the staff time !) 3. Get you the result you want.
SARNs now no longer apply solely to electronic data (if not now, then very soon - I know the change applies this October, not sure of the exact date).
Probably worth doing, anyway, if you haven't already.
If they're using solicitors then serve a seperate SARN on the solicitor too.
C