Well all is good, and I now hold a nice shiny JAA PPL. This conversion from an ICAO license does indeed work and I now have the paperwork to prove it!
The RT license has been issued with a dependency on the Australian RT license, but as I've already mentioned that is just a case of doing the comms theory paper and the test. LASORS wasn't that helpful with the RT info; so I'd advise anyone else planning to do this route to check with the CAA first. Apparently there is this unwritten allowance that Australian and Canadian PPLs with an RT license don't need to do the test here - but not many people at the CAA seem to know that. The person who issued my license didn't - I had to ask him to check with someone else.
All told, it was quite a pain free process, and the people I dealt with at the SRG were all very helpful and pleasant.
Paul.