Got a new one today so I thought I would have a look inside the old one and see why it wasn't working.
What appear to be rivets on top are spiral screws. They may be able to be carefully pried off, I ground them off thinking they were rivets. Once the top is off the rubber gasket and metal cap can be removed and contacts cleaned if you wish . They were quite grungy. You can see how it works.
The float cylinder is pressed and glued on but came off with light judicious prying to reveal and actual water ( brake fluid ) logged old cork ! Not bad for 53yrs of service I suppose.
Technical advice Q&A