Well, I had to make a Farnell order up to £20 for the free shipping, so I ordered some parts to try out these valve controllers. Put together a little dual channel one on prototyping board, and this is what I've got:

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(Just shows top and bottom sides of the board with an antweight motor for scale; currently ~22mm square)

All seems to work fine during the limited testing I've done so far; It'll run a 550 motor at 12v under load without getting too warm, and seeing as it should be good for 8A at 100v I'm guessing it'd fire a Burket valve just fine.

If I did get enough interest to do a production version, it'd likely be ~50% smaller and wrapped in heat shrink. The firmware is also fairly crude (thrown together for testing), so that'll need various improvements before it's suitable to use.