Really sorry about my crude drawings and photos Was in a rush and only had felt tips to use.
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Plug A and Plug B are the plug sockets and not plugs themselves.
I would get two single sockets and a double back panel B&Q is a good place to see the various options available I went cheap.
The power element is a computer power lead with the opposite end of the plug cut off. The longer the better for me anyway.
It's fairly straight forward and hopefully you can follow my diagrams. I'm not an electrician but I think the final drawing will work well.
There is also an option to completely avoid the need of the Euro style connector by running the live direct to the relay and then join the two middle relays pins.
In which case you'd ignore the brown lines in the picture below and use the pink one instead.
This should work and would mean that you could never accidentally heat and cool at the same time.
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Let me know if you need any further explanation or a better diagram and I'll try again.