Its been a while since I was last in the circuit with a microlight - it was at Sywell. I was in a C150 and was no.2 on final to a microlight, who I could see very clearly. Guessed he was tootling along quite slowly so took an early decision to go around. No debate really, and if I wasn't able to see him I would have gone around anyway just to be sure. Can't see how your intentions would have been difficult to understand.
Not sure I would be bothered to go the extent of an airprox, but I would be tempted to do a CHIRP. Also find it hard to see how this can be a problem beyond application of common sense/good airmanship, but if it saves some lives then I'm all for better understanding of the topic.