Salus to Worcester wiring

Hi all. I have just purchased a wireless room thermostat for my central heating. I've come to wire in the receiver to the boiler and found the boiler manual (Worcester 28i jr) calls for three wires, as I expected but the receiver (salus rxrt510) depicts four terminals. I'm not 100% sure which wires to send where. Any help will be gratefully received. I attach relevant diagrs for perusal.
Thanks in advance

Do you already have a room thermostat?

ShaunCorbs said:

Do you already have a room thermostat?

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Hi Shaun. There is a room thermostat on the wall, but not plumbed in to the boiler. It's old and I'm not sure it was ever connected to this boiler at all as there is no evidence of it the boiler end. So short answer, no ☺️

Then as your job requires wiring internal to the boiler eg wiring inside the boiler best option is to get a gas safe engy to install your new stat