The dive doc had some interesting things to say. I was mainly there to get my PADI form signed off (which he was quite happy do do despite my asthma) but I also had an informal chat about some of the issues that have been mentioned here. Spirometry results like the ones I posted above are apparently fine for diving. He did not seem at all concerned about the possibility of hyperbaric injury due to mucous plugs. He said he has never heard any report of it actually happening. That leaves only bronchospasm as the major risk factor for diving (of any kind).
So the take home message was this: dive sensibly. Don't dive if you need to use Ventolin regularly; take regular steroid inhalers such as Becotide instead. Don't dive if mild or moderate excercise is a trigger for your asthma. Don't dive if you have a cold or a chest infection. My personal (non professional!) conclusion about freediving is that if holding one lungful of air is OK at the surface, then it's probably going to be OK at depth too. It can't expand to a volume or pressure any greater than what it was at the surface unless of course, you surface so rapidly that you shoot out of the water and ascend to 1000 feet
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I also agree with what has been said about the usefulness of swimming and excercise in general: maintaining a good level of fitness is far and away the best way to manage asthma. Over many years I have observed that the severity of my symptoms is directly proportional to the amount of time I spend sitting on my backside!
Happy diving to all,
Pete
