Hi Mike,
Thanks for info on car cover but it is a bit outside my budget as I am out of work here in the UK. One trick I did a year ago was put a purpose made tray container of dehumidifier crystals in foot well which actually dried out the air and after a few weeks turned to water in the base but the car interior stayed dry. This simple + cheap solution did actually work just had to change the crystals, which came in a special bag to fit in the tray.

UPDATE - Second hand £15 heater relay module 996.615.117.00 came and I fitted today. Now the heated mirror switch is finally off and goes on via the dash switch. So all now seems to be sorted. Thanks to you all for your help. :)