I now recall that the engineer did indeed say to check that the drain hole was not blocked, and that this (blocked hole) was the probably the cause for over icing and subsequent non-cooling of the fridge.
Although the fridge does appear to be working (maybe my ice clearing helped?), I intend to defrost it completely once we have disposed of the Freezer contents. I understand this will likely take a couple of days to be sure it's done thoroughly. Hopefully, this will be the end to the problem, unless the heater is faulty, in which case, I'll see ice build up again. If this happens I'll see what my local friendly repair guy can do.
Sorry, don't visit here often
PM's read and replied to. Thanks.