I said I'd report back when time permitted....
Have duly removed screws and head unit, which remained disconnected for about half an hour as it happened. Refitted the replacement CD changer then reconnected the head unit and powered it up.
Entered codes for radio and CD changer as requested on display then switched to CD - got NO MAGAZINE first up as I'd not filled it with CDs at this point. Put half a dozen random ones in and replaced in unit in boot. Repowered up radio and switched to CD and what did we get....
ERROR CDCDoes this now mean I have inadvertently Christmas-crackered the replacement set which was working prior to me getting my hands on it? :-?
I'm extremely reluctant to buy yet another one (at upwards of £40 a throw) to have a repeat occurrence. Given that I cannot get the changers to work it also means I cannot sell them on as being in working order. Does this point to a more in-depth issue with the audio system and will an auto-electrician or audio specialist be able to accomplish anything that I haven't tried?
Taking it to Vauxhall is a definite no-no given the exorbitant costs involved (and VAT goes up in January

) and I imagine that ripping out the entire audio system and replacing it with aftermarket kit is not going to prove cost-effective. Given these scenarios then I think once the weather improves - July perhaps - it'll be time for a float on ebay....