I have been spending increasing amounts of time on the British Microlight Aircraft Association (BMAA) forums, where I have been picking people's brains about instrumentation. Jeremy Harris, Steve Wilson, Paul Collins and Paul Dewhurst have been especially helpful. I like the BMAA Forums as it feels like a community and everyone is very willing to share what they know.
I had no idea that most compasses on microlights were regular, common or garden nautical compasses, or that digital compasses were not just a gadget of little real, practical use, though Jeremy says the compass on his watch is "pants" because it has to be held perfectly horizontal to work.
Paul Dewhurst (who has won international competitions in navigation) has even used a flat walking compass (the type with a bezel) and managed fine. He said there is nothing to stop me just fixing it to the keel (I could probably just use a bit of black duct tape?). Steve Wilson, who flies near me, flies an unfaired Dragonfly with a Silva kayaking compass strapped to the keel, which is another very affordable option.
Jeremy recommends a Suunto watch with vario and altimeter, but I am not sure this is necessarily going to be cheaper than a Digi Fly. On his advice I have placed a wanted ad on Paramotorists' site.
GPS systems also have altimeters and digital compasses, and one that is very affordable is the Garmin Etrex Vista, and coincidentally Paul has one he is willing to sell. Steve just emailed me to say that you can download your track and see where you went on Google Earth, which is pretty seductive stuff! He also pointed out that an ASI is not really necessary as you know, for nav calculation purposes, how fast you go in trim (with the bar neutral). So, the Etrex could be all I need, though I would like a separate compass that I can just glance at (especially as the Garmin compass uses lots of battery).