vaillant 618

I have a US Nest Learning thermostat that I want to connect to my Vaillant gas boiler (Ecotec Plus 618 - Vu186/3-5 R1). The boiler accepts both 24V and 230V thermostats as well as eBus. Below I am attaching a diagram with the connections as well a photo from my Nest Thermostat Installation...

Only been in this house few months, so haven't got 'acquainted' with our boiler yet. Back from holiday two months ago and spanner was showing and 0.4 pressure. Put on heating to test it and it fired up OK. Relief all round. Two days ago, we go dreaded F22 error and nothing was working, heating...