or vehicles going to a workshop for modifications ?
, this one - used vehicles fitted with tachos going to the workshop for work to be done…they would be private vehicles after the work’s done (horsebox/racehome/camper etc) but I’m guessing that doesn’t make any difference?
If you never carry goods I would say you’re exempt from the tachograph regulations.
However if you do sometimes use a HGV to carry goods either for your own business or to carry other peoples goods it’s a bit more of a grey area I’m afraid.
Here’s a link to the EU clarification note on the subject which I think the DVSA would almost certainly comply with.