Drayton Tempus Seven Timer for Water/Heating?

I'm no expert but recently called in a electrician to look at our heating/water timer (Drayton Tempus Seven). We've complained that timer tends to come on ok, but often (irregularly) not to switch off. Electrician doesn't seem to think there's anything wrong with timer?! But (as usual) said it might be something quite expensive to change. I think its a fuse on the timer as sometimes the thing works ok, & at other times it doesn't. Any advice? The boiler is a Honeywell.