Several things to try with a 'stuck key' as it may be caused by trapped debris.

Turn the keyboard on it's side, long edge facing down and give it a good 'dunt' on a hard surface.

Use a can of compressed air, with the extension tube fitted, and give the area around the key a good blast.

Finish off by using your hoover, with the extension hose and a soft brush fitted, and run it over the entire keyboard.

FWIW, I use this latter process on a monthly basis to keep my Apple keyboards free of debris, particularly dead skin from typing fingers

Thanks - I have tried various things today without success but I have booked an appointment at the Genius Bar for Tuesday just before a regular 1:1 training session. Still just the one key not working.

The wireless keyboard came with the iMac and I took out the extended warranty so hopefully it will be covered - usefully I bought the wired full keyboard at the same time for those occasions when I do a lot of spreadsheet work as the numeric pad is much better than numbers across the top.

In reply to a post by Kaoshan:

Several things to try with a 'stuck key' as it may be caused by trapped debris.

Turn the keyboard on it's side, long edge facing down and give it a good 'dunt' on a hard surface.

Use a can of compressed air, with the extension tube fitted, and give the area around the key a good blast.

Finish off by using your hoover, with the extension hose and a soft brush fitted, and run it over the entire keyboard.

FWIW, I use this latter process on a monthly basis to keep my Apple keyboards free of debris, particularly dead skin from typing fingers

Will buy a can of compressed air tomorrow and try that as everything else I tried and no joy

In reply to a post by lenaspell:

Thanks - I have tried various things today without success but I have booked an appointment at the Genius Bar for Tuesday just before a regular 1:1 training session. Still just the one key not working.

The wireless keyboard came with the iMac and I took out the extended warranty so hopefully it will be covered - usefully I bought the wired full keyboard at the same time for those occasions when I do a lot of spreadsheet work as the numeric pad is much better than numbers across the top.

Well the answer is that the apple care for the iMac covered the provided wireless keyboard so they replaced with a new keyboard without quibble. No attempt to repair or even check that the problem was as described. Can't argue with their service at all. So quite why it decided to pack up is anybody's guess. The new keyboard only requires two batteries instead of the three required for the old one. Oh and they gave me my batteries back without me needing to ask.