Thanks - but will it be simple to wire in? I've been told that if
I use a different clock it would need rewiring.

It sounds like you have very simple wiring as it is.

I bet you have one permanent power cable (the red/black motor one) and one
switch cable (purple switch one).

Almost time clock with "volt-free" contacts should be simple to wire up.
Connect the purple wires to COM and NO. Connect the red wire to L(ive) and
the black wire to N(eutral). (Unless the old one is a low voltage thing, but
I doubt it in on a 37 year old boiler).

Christian.