I only know what both the VI (as was) & the local TC said geebee.
Whilst we were driving for the lease company, the vehicle was possibly out of service & the fact that the people we were working for did not require an O Licence was a key issue, as I understand it.
We were all self employed & we kept our own tachos, with VI agreement.
The situation was that neither the local TC nor the VI could tell us why we should use a tacho, or which rule exempted us, it was in their opinion a “grey area” & one they informed us they would be happy to test in court if anything went wrong & we had not abided by E.C. regs.
The recommendation was that we should use a tacho, but that is all it was, a recommendation & one based upon good (& safe) practise rather than any clear legal requirement to do so.