Well, it seems bsa-bill got it right first go, although how he knew that I had three bulbs with faulty/blown brake filaments is beyond me! Well done Bill.
I checked all the wiring, dismantled the switch, cleaned all connections and earth, but got nothing despite trying three different bulbs. Then in desperation I tried a 12v equivalent from my car bulb kit and bingo! It worked. Not bright enough, but at least I knew it worked. Off down the shops for two 6v 21/5w and both work.
Thanks all for your advice.
Paul