R.D.
I know a recent instance of an ATCO in the same position as yourself, who joined a radar equipped unit.
Said unit carried out a programme of simulator and practical training that had been previously approved by SRG, which proved successful, and the candidate then undertook MER and validated.
The SRG requirements were very stringent, and had the person involved not met the standard, they would have had to do the whole course again, (at their own expense).
Unless you can find an employer to do the same for you, your only option would be to do an assessment for prior competence, (which Shearwater and the mob at Rudloe Manor, whose name escapes me, certainly can do, I don't know about CATC), and hope to get a maximum of 50% dispensation from doing the full radar course.
I know the full course is very expensive, but after 5 years out of it
doing anything but the full course would be a real challenge.