Stripped out parts of the centre console to access the lighting rheostat. Earlier had established that several of the lighting bulbs were OK by removing them and testing them a battery charger. It all came apart easily. Also found that the 2 outer heater levers were not connected to the controls - reconnected them.
On trying to start the engine/test the heater nothing happened - battery completely flat. Off to Halfords to get a new one, they have nothing suitable. Home to do some research, Beans on the Club Triumph forum came back very quick with a suggestion of what battery to use. Unfortunately the Bosch S4 023 is not available locally.
Allwoods http://www.allwoods-automotive.co.uk/ my local independent car parts place. had an Oldham advanced battery (3 year calcium chemistry) for £47.99. Apparently the same size as a classic mini and getting difficult to source - several makers no longer produce to that spec - I was told.
Alternative battery size 015, 038
Will take some pictures of the dissection tomorrow.