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
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