Just to update you all.
Now have the car back (picked her up tonight) but the problem isn't sorted.
Basically on a test run today, with the convertor out, Envy discovered a strange lack of consistency....
Using a GPS to verify actual speed, at 25mph the car indicated 30 (so over-read) but at 75mph the car indicated 60 (so under-read). Totally stumped for solutions now so thought may as well get the car back and fit a Road Angel or something to give me my speed details.
Not really the outcome I wanted but guess it'll suffice for the time being.
Oh, and by the way, a big thanks to Gaz Walker for the significant reduction in cost compared to the real amount of labour hours.