I have just converted my Y plan to S Plan. All seems to work

However the water circuit has never been 'right'

There is an unvented OSO Cylinder in an airing cupboard (in a bathroom)
the other side of the house to the boiler.

It has a zone valve controlled by a different circuit to the boiler /
time switches etc.
There is no call back to the boiler.

So the time switch comes on for the hot water and pumps to that zone.
The zone valve cuts in and out (on a different circuit) and the bypass
valve deals with the fact that there is no call back from the valve to
the boiler.

SO

I thought I would run a wire from the time switch to the unvented tank.

Looking at the standard s-plan wiring for the zone valves the Grey wire
appears to be permanent live and the Brown wire is live when the timer
calls for the zone to switch on.

Two questions
1. Why does it need the permanent live through the grey wire?
Can I join the grey to the Brown?
(I can send four wires to the otherside of the house that way)

2. Do I need to get this 'certified' because of part 'P' ?

Thanks

Mark