Can't turn central heating off

Good Afternoon
Hope some one can shed some light on this.
My boilermate 2000 central heating is constantly on.when I turn the timer off heating stays on. Turn the thermostat down to zero and it stays on. Iam sweltering but my other half loves it (she doesn't pay the bills). I have turned it off on the mains and back on, on the advise of gled hill but still to no avail. It sometimes goes off but not at the time set on the timer. Is there any diagnostics I can safely do to narrow down the fault. Can't seem to find a local plumber to help
Mention boliermate and they seem to go all disinterested.
Many thanks
Simon

ACB
Has failed
Common problem

Mike
MGC/H2O

Hi Mike

Thanks for that

It seemed to behaving itself yesterday, coming on when it should and going off when it
should but then about 4 this morning it was blazing away even thought the timer was set
to 6.30. I will get a new ACB and report back

TA

Simon