Interesting stuff.
I'm suitabley convinced that the Honeywell Open Therm bridge will do its job properly.
Fairly confident that my Vogue boiler will too given it has 2 connections labelled 'OpenTherm', although not sure on the detail as there is no real mention of Open Therm in the manual whereas there is a lot of detail regarding the affect of adding an outside temperature sensor. For example the manual explains that the HW heating flow temperature can be set independently of the CH flow if the outside temperature sensor is fitted (and will override the CH flow temp if there is a simultaneous HW and CH requirement). Hopefully the same would apply with Open Therm (bearing in mind I do not have the Evohome Hot water kit) and that HW heating flow call from the zone valve would override any Open Therm flow temperature adjustments made from a CH perspective.
Think I will go with it and will report back my findings.
I also ordered the outside temperature sensor yesterday as it is quite cheap.
Not sure however whether the outside sensor and Open Therm will work together, override each other, cause general chaos or be the best of everything! Anyone got experience in this respect?