Can anyone tell me how much plume I should expect to see ?
In cold weather would I expect to see a plume all day, or
just when the boiler is starting in the morning ?
Would there be a plume in summer ?

My Ideal Icos plumed any time it was on. It would certainly be better if the
flue terminal had been above the roof line.

Vaillant EcoMax 622/2E

Not directly, but I hear it is good.

Megaflo HE CL210 Indirect Cylinders

My parents have a Megaflo. It is excellent. 210 litres is seriously large,
enough to run two full baths simultaneously even with the boiler off. Are
you sure you need that much?

Christian.