Central Heating Relay help needed

[connectrix]

I have an old E type boiler that has a gravity fed water system , heated by the boiler then stored in a tank. The central heating just has a pump. I have removed the old room stat and programmer and installed a new 2 channel programmer and room stat.
I thought it was all wired correctly as it has been that way for years but when connecting the new programmer etc up I can see a problem. Basically you can heat the water on its own fine but if you want the central heating on you have to have the water on at the same time to fire the boiler. So you cannot just have the central heating on its own as only the pump runs but the boiler is not firing. From what I can see the only thing to do is link the central heating pump to the boiler suppressor so they work together when the room stat requires heat. However this will mean that if you only want hot water then the central heating pump will start running so I was thinking of installing a contactor/relay (feeding the coil from the room stat) so that the boiler will fire when the central heating pump is running and the hot water will work fine without running the central heating pump??
Can someone please confirm this is correct and advise me of the best contactor/relay to use and where to buy as I has a look in the RS catalogue and there are various types etc...
Obviously I know I need a 230v coil and probably din mounted with screws. I am familiar with contactors but not used relays before (I know they use the same principal)

 

[connectrix]

On the pilot or does the boiler suppressor have to be fired?

 

[micknew]

Install a 2 port valve on the HW input to the cylinder and fit a cylinder stat to control it.

 

[guest119]

Strap on stat opens and closes zone valve on the top coil connection. Stat has switched feed from channel one to open valve, spring return. Valve supply looped to valve micro switch core grey, core orange to boiler.

Zone valve also on heating line close to boiler, either flow or return. Channel 2 via room stat opens valve (spring return) and runs pump. Valve supply looped to valve micro switch core grey, core orange to boiler.

Separate, independant HW and H with gravity HW. Pump will run as valve starts to open, not like a genuine S plan.

Boydy