Thanks for the suggestions guys, especially the extensive post by His Dudeness.
I have limited time today, so here's what I've done today.
Firstly swapped over the hoses on the carbtune and I still get the high reading from the left/nearside carb. It's right at the top of the meter. So It's not the meter.
I've pulled the carbs off. I can't really see anything wrong, but bear in mind I'm new to working on carbs, so please treat me as someone with no knowledge.
I had a look at what happens when I turn the balancing screws and found something interesting. I'm guessing that moving the balancing adjuster screw changes the angle of the butterfly in just one of the carbs? Please confirm or deny this.
Looking at the original carbs, plus the replacement from the ones I bought from Deefer666 I can see that when turning the balancing adjuster screw for the two left/nearside carbs, the butterfly in the carb on the inside of the left/nearside carb moves, but the outer one doesn't appear to move. Looks the same on the other two carbs, just one butterfly moves.
However when I do the same thing on the replacement carbs, with my mangled carb on it, I see that the butterfly in the left/nearside carb moves, but not the one on the inside of the left/nearside carb! That confuses me, as it's the opposite of the other set of carbs. Surely the same butterfly would move? If only one butterfly moves when making adjustments between two carbs then perhaps I should swap the carbs back and just install the replacement carbs from Deefer666?
Hope that all makes sense.