Its off the on/off switch mate, it may still work but it will not switch off and the battery will run down. Think it will need a new switch, there on ebay if you know some body who can solder. But as above Jim is your best bet for repair, lads really rate him and his prices are very good. Have a look in calibrating Mk1 posts at top top for his details and web site.
Cheers Arry