Can I use any type of small expansion vessel here? (For example: the
smallest of these: http://www.bes.ltd.uk/products/109.asp)

The manual states that an expansion vessel should be installed if
there are any backflow prevention devices in the line - including
water meters.

I don't have a meter at present but that obviously might be a
possibility at a later date...so I guess I should fit the vessel.

Mr F.