KwikPhix you are quite right mate, this is a messy procedure and unfortunately logic and even precedent do not prevail. The advent of JAR-66 has allowed the CAA to put the onus of certification privileges pretty much in the control of the JAR 145 organisations. Whether we like it or not its here to stay so we have to make do.
In my experience converting from Section L to 66 with restrictions can inhibit your authorisation potential and lead to confusion. It really depends on how the 145 organisation you are working with have written their MOE/MMOE with regards to certification privileges. No two 145's are the same, you and I both know that, as yet, JAA/JAR has not created the utopia of a level playing field. Most 145's follow the road of black and white purely because no NAA, let alone the CAA can get a handle on this mess. In short full B1 / B2 with no restrictions is currently the most straightforward option.
I assume that your'e after a B1-1 Aeroplanes - Turbine rating. On this basis and i.a.w. current CAA guidelines to achieve this you need to pass the following JAR66 modules :
3.9 to 3.18,4,5,7.7,11.5.2,11.6,11.8,11.10,11.14, and the two you have already sat, 15 & 17.
In addition check on your " protected rights " status. For example if you held Avionic extensions in LRU category which were valid on 1/6/2001 and you were currently employed within a 145 organisation at that time it is possible you can be exempt from module 5. If you qualify for this it needs to be backed up by a letter of confrmation from the 145 concerned.
My interpretation of the experienced mech issue is that if no formal NAA/CAA licence qualification is held then you probably have to start from square one. I have made cases for fellas before and achieved some small exemptions but this is done on an individual basis. As has been so well publicised, the ability to talk sensibly on a one to one basis with engineer licensing is proving more and more difficult.
We could go on for hours about this but I do believe this is the most painless way currently available to us A/C/X/R Section L blokes. Forgive me if I have only told what you know just trying to help. Good luck.