Just to complete this post (hopefully!)
The replacement sensor arrived a couple of days ago and was duly fitted. The washer fired straight up and has run five times fully since then.
I don’t think why this all happened will ever be known or why a failed sensor could be linked to a failed TNY both happening at exactly the same time. No other faults were found on the boards or elsewhere.
Still (if it continues to work!) a TNY at £3.16 plus a second hand sensor at £6.50 is a hell of a lot cheaper fix than the £500 plus for a replacement dishwasher my wife wanted me to buy.
Hope the post may help others, and thanks a lot Dave for all your help.
Regards
John