Thought that this might help sort some of you out, who like me are having problems with brakes.
I asked the tester via email and here is the reply:

We add all the footbrake figures together from each wheel, then divide that by the calculated weight of the vehicle and then multiply by 100. This gave you an overall result of 50% efficiency. Now this is fine for an MOT but we look for 60%, however if more that 2 of the 4 wheels lock out then you automatically achieve a pass result regardless of the figure gained.

Yep, very helpful. Ta.

any two wheels or the front two locking?

It does say "any", so I guess it does not matter