3 Point,
Guidance on Differences training required is clearly specified in LASORS. Section F Appendix E.
The only pilot training that has any solo element is the PPL or part of the integrated CPL. All other courses of training are dual only.
To make my previous comments as clear as possible, to send a pilot solo in a tailwheel before you have signed off the differences training, you must ignore that they are a licence holder and act as if they are a student without a licence. That is why a licensed airfield is required. It is bending the rules but does work if you feel the need.
If you claim that they can solo anywhere because they are a licence holder etc makes them fall foul of the following;
"In order to fly as pilot-in-command (PIC) of an
aeroplane in the SEP (Land) class you must have an
SEP (Land) Rating endorsed on your licence." - LASORS.
There is also the requirment to have differences training endorced in the logbook.
As for training for lapsed class ratings - well actually the training is at the discretion of the licence holder so that they can pass the test. They can choose to complete no training if they feel they can pass the test.
Remember also that the insurance especially in regard to private aircraft may not allow student solo flying at all i.e. the PIC will always have to be qualified.
Regards,
DFC