IN 2015/034 stated that if an instructor had FCL.945 privileges included in his/her instructor certificate, then the same privileges would apply to SEP, SSEA, SLMG or TMG ratings included in other UK-issued pilot licences (UK PPL, NPPL, JAR-FCL PPL).
Unfortunately the IN was cancelled earlier this year; however, the CAA has confirmed that the policy still applies.
So the answer to
I have a CRI rating with FCL.945 privileges on my SEP and TMG. Can I conduct the hours flight with a instructor and sign the licence of someone who has a SSEA and SLMG?
is YES.
It will go into CAP804, I understand - so at the moment you won't find it written down anywhere. But it IS the current policy.