On Tuesday, 3 March 2020 21:50:42 UTC, Chris B wrote:

(he mused on why they might have blanked off
the regulated cold supply on the main tank PRV).

That does indeed seem odd. Balanced hot and cold mains supplies are a key benefit of unvented systems. You wouldn't really get that with separate PRVs (although you could get arguably get close enough).

Original plumber has agreed to come back and do the work described as
required by the tank manufacturer, but having looked much more into how
these systems work I thought that a small expansion vessel would also be
needed in addition to the PRV.

No expansion vessel is required on the cold as there's no expansion to accommodate. The only reason is it required on the hot side is due to the expansion that arises from heating the water.