Hunting Idle and Misfire

[David]

Again thanks for all the helpful info.

I took the car to a local MG Rover specialist and it is indeed leaking air....

Have they said where? One of the most common air-leak sources is the PCV system, since the hose or rubber end elbow-joints tend to rot or harden, more so on earlier cars (which yours is I believe). The valve unit is a 90k mile replacement service-item.

If it is that, note that the padded pipe/hose and PCV valve are Mustang parts not MGR, but available from the USA or maybe from a UK Mustang specialist.
Part-numbers here: https://www.two-sixties.co.uk/mgoc26...valve-and-Hose

David.