If the timer is a 103 you can buy a modern replacement 103e, this slots straight onto the old backplate without altering any wires.
Ian
September 2008
most combi heating wiring is a live out of boiler into time clock then a live back to boiler when time clock demands so yes it should be a pretty simple swap.
generally timeswitch will have a common connection (live) and a load connection (switched live).
iain
September 2008
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.