Wiring in a Honeywell L641B Pipe stat

[Steve1309]

For some reason, a pipe state was not wired in to the frost stat. Consequently, when the temperature in the garage where the boiler is, gets below 5 degrees, the heating comes on and doesn't stop. I can see that the pipe stat needs wiring in series after the frost stat but not sure how to do this correctly in the Honeywell sundial wiring centre. I've attached a photo of the wiring centre as is. The frost stat wire is indicated with the red arrows. The Frost stat is mounted next to the wiring centre, The boiler is about 5 metres away. Any advice welcome. 
I have tried to get a heating engineer out to do this but it seems like something nobody's interested in doing. TIA

Edited December 9, 2022 by Steve1309
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[ProDave]

You will have to disconnect say the blue from the existing frost stat and add the pipe stat in series, requiring an extra terminal, e.g. a single choc block to make the joining connection.

[Radian]

The conventional arrangement is a bit weird to me. A frost stat trips on low temperatures to protect a boiler located in an unheated space. Fine, and some boilers incorporate this sensor inside the boiler. But external sensors won't necessarily be heated by the boiler, like if they're 5m away. So why is the conventional solution to this to add yet another sensor (pipe stat) on the return flow instead of just mounting the frost stat on the return in the first place?

 

[Steve1309]

Thanks @ProDave

You've confirmed what I thought. Cheers