Shouldn't be too bad, 30-40 years old would probably mean a 15th edition install, bonds might be 6mm, but cabling wise, it'll all be as per modern stuff except for the colours. 2.5 has had 1.5 earth since 1981 ish.
You'll likely have a lack of RCD to domestic lighting, concealed cables and bathroom circuits, maybe even to sockets. MCBs might be to 3871
Jot cable and breaker types and sizes down, turn ciruits off and identify whats what while doing a bit of a walkaround to externally inspect for damaged accessories, unplug appliances, isolate things like boilers, trip off RCD accesories, and identify places where DIY bodgers and kitchen fitters might have had a go. Isolate, prove dead, link live and neutral in board, do a quick IR to earth after checking for lack of continuity, test on 250v at least initially, only test at 500v is result at 250v is good and you are happy you have removed sensitive kit. Take rings out one at a time and do end to ends, put back in. Go and do covers off visual, focus on any areas you identified as suspect before, aim to do 10% of accessories. Covers back on, check connections in DB, Do a Ze if you can, if not just get a Zdb, power back up, test RCDs/RCBOs, DB cover back on, go and loop your cirucits. If you test at a local isolator (i.e. cooker) remember to do continuity test between iso and appliance, in the case of a cooker, the oven hinges are a good place to test continuity. Quick walkaround to make sure everything is back together and check you'd plugged the freezer back in.
If in cambridge, then you should do alright, just undercut Jordan/Artisan who seems to take four sparks and a camara crew to a doemstic EICR....
