MJR,
The conversion of BCPL to JAR CPL(R) is in white 19.
It says 5 hours training in a complex type (eg arrow) plus a Licence proficiency check with a CPL Examiner. The test is not via the CAA, you arrange it with the examiner(I am being charged £155). The 5 hours does not need to be approved so any Arrow training is valid and the test is like the CPL test but without the Nav or instrument flying ( takeoff, steep turns 45degrees, emergencies, landing)
You also need a JAR class one medical and the CAA want £170 to convert your licence.
All in all a good deal to avoid the Jar CPL (£5000) and another CAA test (£550) and the JAR ATPL exams (another £3000)