I'm not a domestic sparky but I would consider myself competent to complete this work safely. I don't have tools because I'm here on holiday from Cape Town and staying at my sisters house, I didn't bring any tools and unfortunately she doesn't have any either. If I really need tools I'l buy some but I'm just loathed to do that for one job.
I've been back there today for further investigations and I repaired a couple of the light fittings and also replaced the round metal switch in the coal shed with a new switch and backbox. The wiring went inside the backbox with almost no disturbance to it but it was a bit short. Rather than trying to massage the VIR to fit I ended up extending it a couple of inches.
I removed the locking rings and face plates of the other switches, they were fairly easy to unscrew. The plate holding the switch modules is brass wood screwed to the backbox so I backed all the screws out a couple of turns each and gently pulled everything forwards a centimeter or so. The rubber wiring is still in tact and doesn't look degraded, I'm pretty certain I can undo the switches and remove them without damage. I think I'm going to repeat the proceedure of extending the VIR wires without disturbing the way they are laying and put a new surface mount PVC backbox over the top of the old flush wooden one bringing the wire extensions into it through the back. If that goes well it's just a matter of fitting any suitable new SELV switches.
The end result will be surface mount switches but I'm not going to lose any sleep over that.