Perhaps not as bad as it first appears. The instructions you posted #17 does say it has both internal and external sensors, so you would need the 'internal' one. See if it makes clear how to switch to this. The actual wiring is tricky, tho', since it isn't 100% clear (actually, not even 50%...) how terminals 1,2,3 and 4 are wired internally. 3 and 4 are simple enough - that's N and L (which you currently don't appear to have). Then there's how to connect to the 'switching' terminal, the one that'll turn your boiler on and off. The diagrams are contrary. Post #3 suggests that 1 and 2 are an isolated switch, but the later diagrams #18 suggests a live feed to 1 or 2 internally. Sorry, I'm stumped.