Thanks a lot!!!!Look here and at item 30 as exemption . https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/754827/declare-heavy-goods-vehicle-does-not-need-tachograph.pdf
If registered as Motorcaravan then has class 4 MOT and no ned to claim exemption . If not so registered then subject to plating regs and will have HGV MOT and at test you fill in this form and self certify item 30 .