Try sending your letters recorded delivery. Generally it is bad press if you don't reply to those letters as you have proof how many that have been sent and when.
Also is this not a matter for the highways agency as well?
A reciept of postage like recorded delivery I believe can be used in court as evidence.

Rgds
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I've e-mailed Meridian TV as it lies in their domain. If enough of us do that then you never know.....