Thanks for all the advice on this forum.
I have now plumped for replacing the reversing light with this item 1pc CREE 1156 BA15S P21W Xenon LED CANBUS REVERSE LIGHT BULB WHITE HIGH POWER 261169683340.
There are several like this ranging from£22.50 to £9.90 to £8.95. I went for the latter as they offer a refund if it doesn't work out. They are one of several companies that sell in the bottom range on Fleabay. They are all the same!
The bulb's circuit board has CREE on it but whether it is a CREE or not it is rated at 380 Lumens
I can only say that the difference is like night and day. Massive improvement so that I can recommend this minor upgrade without reservation.
Only regret is that the glowworm was evicted.
PS Don't forget to check the operation with the boot lid shut.
I'd post some pics but photos of lights are notoriously unreliable and misleading but if you insist I could put up some before and afters