I've been trying this out but I'm having some problems.
Whilst the loop is applied to the VR33 OT connections then the boiler works fine in response...
I'm just using the circuit shown here https://ihormelnyk.com/Content/Pages/opentherm_adapter/opentherm_adapter_schematic_o.png and also the same here...
Unfortunately it turns out he is in Ukraine and unable to supply these at present for obvious reasons. He was kind enough to refund my payment right...