Gaz - John Sleath will show you an at flywheel figure too, but he's reluctant to stick it on the print-out - possibly to avoid anyone accusing his dyno of under/over-estimating the transmission losses.
As I see it, the main benefit of Aldon is lots of historic data for comparison purposes - though Club G40 were at John Sleath Racing last year, so there's some info there too. In favour of John Sleath Racing is multiple runs - and John's often happy for you to swap ECUs (to try a different chip) or tweak the ignition timing slightly inbetween runs.