Ok,
so it seems that I need to go like for like. I guess that my car's road speed is calibrated to the 140mph speedo head - so going with a 120mph unit is not an option.
There seem to be a number of different versions of each unit, For example, I have found double plug versions of the 120mph unit with part references 191 919 035 BM, 191 919 035 G and 191 919 035 CT...and presumably there are other variations as well. In speaking to a breaker this morning, I gather that these represent differences in the frontal appearance as well as the orientation of the plugs etc.
Does any one know whether I can fit any double plugged 140mph unit to my car or do I need the exact specific part number? I'm a little concerned that, with these parts becoming harder to come by, the chances of finding the exact match is reduced. By the sounds of things, this appears to be a pretty common issue - so I guess people are finding a way around it.