After a period of time, if I then subsequently pass the CPL written's can I then start instructing for the full PPL without any further training
From the way FCL.915.FI is worded, it does appear so, but it is not crystal clear. It says that when you
apply for an FI(A) as a PPL(A), you should meet "the requirements for CPL theoretical knowledge, except for an FI(A) providing training for the LAPL(A) only".
This suggests that if you subsequently pass the CPL written, you don't need additional training, but you do need to re-apply for the FI(A) certificate.
Check with your national authority!