Further update for people who are as lazy as I am and/or too stuck-up to stand in a Post Office queue (also as I am!). It's very easy to do this by post, I have successfully done this.
All you need to do is post 3 pieces of paper to the DVLA, Swansea SA99 1DZ
1) Complete V5 document, with nothing else written on it other than "HISTORIC" in the "tax class" section of the blue "change my vehicle details" section (Section 1). No signature is needed.
2) V112:
https://assets.publishing.service.gov.u ... om-mot.pdfwith "R" written in the category box, name, reg, signature.
3) V10:
https://assets.publishing.service.gov.u ... le_tax.pdfSection 1 - your details, Section 2 the reg and "HISTORIC", Section 3 - 12 months and the first day of the current month - again no signature.
That's it. Just those pieces of paper, no covering letter, stick a stamp on it and post it in a postbox. That's all I did and it took 2 weeks to process. I was previously on SORN, but if you currently have tax paid on it they would then send you a refund cheque.