ummmm is totally wrong on this! Whatever you do, don't ignore it. Now they have found you, it will not go away - only get worse..
As already said, it looks as if a Court decision was made in your absence (because you never got the documents - having moved) & the chasing now is for the payment. There should be a procedure to get the Court decision revoked or set aside on the grounds that you had no knowledge of it & that if you had known you would have defended the matter & the outcome would very likely have been different. The Court office should be able to advise you on this but - if you need to - try to get help from a money advice service such as the CAB.
Incidentally, were you living in a house at the time or in a flat? If the latter, it would presumably be much more difficult for TV Licensing to have any certainty about whether there was a TV in your flat, as there would be signals emanating from the other TVs in the block.