expansion vessel positioning

I had to do exactly the same thing. The vessel in the boiler was probably just about adequate but I don't like marginal things, so added an additional vessel in the loft where the header tank for the original unvented system was located. I have the filling loop connection there as well because there was a convenient cold mains supply there, whereas their wasn't at the boiler position.

It doesn't matter if there is too much expansion capacity. You can calculate roughly on the basis of 10litres per 100litres of system capacity. If the existing one is working OK, then I would simply add in an 8 or 10 litre one and done with it.

It should be in the return feed near the boiler, but as you already have one there, I think you could put the extra one anywhere you like. The one in the boiler will cope with ensuring there isn't a momentary pressure drop at the sensor when the pump starts or a

Thanks guys!

theres plenty of space in the eaves just where the pipes go down to the original system so I'll put it in the return there. Thats a relief - I had nightmares trying to work out how to get it close to the boiler when the boiler is fitted tightly into a kitchen cupboard! One thing I forgot - I take it it is just teed into the return pipe? Thanks again

Yes. I might include an isolation valve so you could change it in the future without draining everything down (although I didn't think of that when I fitted mine;-).

If you're shoving it in the loft, I would put it under the loft insulation so it isn't likely to freeze, although you don't want it hidden either.