We had this once, the old man got a few motors, usually declares em sorn over winter, one of them, he refunded the tax, on that form you have to declare SORN also, so really they were bang to rights, they said, they didnt recieve the decleration, £100 fine, straight away...

WE defo filled it out when got refund, phoned up, told em usual, we did it blah blah, always declare sorn etc

Had to put it in writing to the name of the person on the letter, didnt hear anything after that, till got a new Sorn decleration reminder thingy through the post..

As you say, I would write to them, giving dates etc, but pay it also, then you may get it refunded back, but if they prove you are at fault and you dont pay it, could become costly