Triton T80 electric shower problem?

My Triton T80 has recently started to cut off after only a few minutes of operation (water does not run over hot, but it stops working). Also water continues to run when it is turned off at the unit - I have to turn off the electric supply before it will stop?

If the unit is then left for a while and turned on it will continue to operate as normal for a further 2 to 3 minutes?

Any suggestions - unit is quite old - 10 to 15 years!