A further update -
I decided to get a combined speedo/ revcounter instead of two separate units. I bought mine from Bandit Mania (Suzuki Bandit specialists in the UK).
For £75 I got a backlit LCD unit which can display speed (switchable between MPH or KPH) two trip counters, total mileage (not resetable) ride timer, clock and a rev counter which is a 'bar type' display, and can also be displayed in numeric values instead of one of the timer/ trip sub-displays. It's 'E' marked, and should be fine for the UK MOT test.
It was pretty straight forward to fit, and even you Baggy owners without revcounters should be able to wire it up to show revs (the Mastiff already has a signal wire for it's revcounter).
Too early to tell how durable it is yet, but I'll try and post some pics of it on the bike later this week