Mira 439.78

A: Yes they are a set size and thickness.

http://www.showerdoc.com/shower-spares/mira/MIRA-439-88-Mira-Seal-Pack-(components-Identified-‘b’)-439-88

If the originals are damaged or lost then the above are what you will need. Fitting the "O" ring to the tank first is the best way to fit this part.

A: 

Removethe front cover and with the power off try operating the on off switch, if itis sticky then it will need replaced if not have a look at the front cover tosee if there is anything causing the problem.