Hi chaps, just reporting back how things went and to thank all of you that gave helpful information.
Replacing the voltage regulator in situ is what I've done, and it has sorted out the issues with now a healthy voltage over the terminals of nearly 14 V when the alternator is running, the engine idles nice and smoothly and all electrics are behaving so far.
Especially grateful for drayco's suggestion the alternator could stay in place as it turned out not too bad of a job.
The only thing causing lots of swear words when removing the regulator was one bolt with philips screwdriver head and a #7 socket option to loosen but the space too limited to use the socket and the screw head immediately got ruined.
Eventually just hacked myself some space for the socket by taking a chisel to the voltage regulator (the advantage of getting stuck undoing something you're going to throw away
) and replacing the bolt with one with an allen key head when fitting the new one.
Found fitting the replacement part a walk in the park which I really wasn't expecting but I think I just got immediately my position under the car right and managed not to completely ruin my vision when getting my hands past the driveshaft towards the alternator.
Anyway, I would definately recommend doing this with the alternator in place, just stick your phone up there and take decent pictures so that you know exactly what the layout is.
It's a simple job, but awkward to see what you're doing...
The youtube video ardi posted is extremely helpful and it really is all there is to it. (apart from replacing the whole regulator instead of the brushes only)
Thanks again chaps, I got this sorted really cheaply (46€) and I don't really have a trusted garage around here so they'd probably all tell me to replace the alternator entirely and try to set me back 10 times that amount.