Its not £1.50 a minute. Its got to be cheaper & quicker than calling a plumber or electrician out who doesn't know what there looking for.
A 2 minute phone call could be all it takes to get your waterguard back up & running. That sounds reasonable to me.
If you are having problems with your waterguard you should phone there help line ( low cost) 0871 2882238.
It sounds like the flow switch is stuck open, which means its telling the control panel that waters flowing all the time. Their may be something clogging it up, a clean out should sort it.