Gledhill Boilermate II Overflow

[garlanj]

Help! My Boilermate II regularly overflows (it's not continuous but generally in the morning when people are having showers etc and the hot water is being replenished). The cold fill into the overflow has been turned off but there is still an overflow of (hot - I think) water.

Does anyone have any ideas?

 

[HarrogateGas]

Sounds as if the mains pressure coil inside is passing.

 

[garlanj]

I take it that's serious? Question is though why does it only do it some of the time - I'd have thought it would be a continuous overflow with something like that?