Eureka!!
James, you are a gent, no need for you to swing by now but the offer is very much appreciated.
Having stripped off as much as I could do I was left with the prospect of taking the wheel (airbag etc) off, which I wasn't happy about. So I decided to investigate some more before taking anything else off.
I engaged the steering lock with the key out, then tried to move the wheel - didn't move an inch. So I wondered whether I was being premature in blaming the steering lock?
A little more poking around and turning the wheel revealed that the large black plastic ring that has the airbag etc plugged into it was moving slightly whenever the steering wheel was catching. Further fidlding round resulted in the wheel jamming, my forcing it and a small something pinging into view, which I coaxed out with my fingers.
Turns out there was a small piece of black plastic floating round in there that was getting caught up. One of the plastic indicator cancellation things had disintegrated at some point in the past and presumably this piece of plastic was lodged somewhere until it made a bid for freedom today.
So my panic is over, it wasn't anything too serious after all.
Once again, many thanks for the input and especially to James for the very kind offer of swinging by for a look.
