Well! It would seem I was way out about my failings. I had really thought that I'm OK on multitasking and info processing and if anything let me down it would be my levels of physical capability.
Judging by the feedback I just received, it would appear that I'm an average handler and information processor and below-average at dealing with a high workload. They seem to think I would struggle with a busy cockpit. However, this really does disagree with my experiences, well, all my life really - including learning to fly, when combining flying, navigation and RT crossing a busy MATZ, for example, was less of a problem than getting to grips with the final stages of landing.
I suppose the test must look for something specific to the commercial flight deck, which, not having experienced, I don't know about. Or maybe it was simply that it was the last of the tests and, not having had time for breakfast (which I NEVER miss usually!), my blood sugar was getting low by then. Who knows!
I guess I should rethink my plans on the basis of this.