timer switch?

If its the one built in its quiet starlight forward to replace.
simply unscrew the 2 screws on either side of the front panel, then lift up the front cover and the panel will flip down.
Unscrew the 2 screws holding the clock in from the back and simply pull of the 4 electrical connections from the bottom.
re-fix in reverse order
Also turn the boiler electrics of before starting this procedure

ka23
February 2015