Central heating timer problem

[jelly]

have a Honeywell timer(ST772) running my central heating system.Now it has just started playing up.Most times now when i put the heating and water on together the timer turns off and i get a blank screen on the digital display.By pressing it's reset button the display comes back on the starts heating the radiators and water.As you can see it becomes a pain as this resets the time and i have to set everything again.Funny thing is this used to happen about 4 years ago when the central heating was first put in and the plumber had to change the timer twice (at the time the plumber told me they were a faulty batch)but hasn't happened since until now.
Have people had problems with these timers.
Could it be something else wrong with the central heating
How easy is it to change a timer.
Thanks

 

[kevplumb]

try resetting it

 

[jelly]

I do reset it constantly as the digital display goes blank

 

[kevplumb]

its f****d

 

[doitall]

does that mean it needs a new one kev

 

[kevplumb]

technical term m8 you wouldnt understand