So it appears that you can just use the one valve for your S-Plan system.
You just connect one switch wire to permanent live and the other to your
boiler call for heat, in parallel with your heating valve.

Christian.

OK

So back to the original question

The wiring diagram for S Plan shows permanent live (on grey) and
switched live from the timer (on Brown)

My Question was is there any reason I can't join the Grey wire to the
Brown?

Anyone?

Thanks

Mark