The IR3899 looks awesome and I had not appreciated that the TL494 doesn't have an inbuilt reference - thanks.  I'm still minded to try to roll my own software converter though (for 0.25V).  Last night I connected a 100W 660nm LED to a 24V supply and watched as the current rose from 4 to 4.3 Amps before I bottled out - later I got the spec from China and it said the Abs max rating was 3.5 A but it still seems to work fine.  It was attached to a 30x14 cm heatsink but perhaps it got to 55 Celsius - uncomfortable to hold so I'm going to need some fans ...

Keep up the good work - what you're doing is very interesting to hear about.