Right then... tomorrow I will take the lid off and have a look!
I've been doing some calculations... with a 4mm TP screw (or an 8TPI Acme, if I can get imperial anywhere), I need around 576rpm to achieve my desired 600mm rise/fall within about 20 seconds. If I can get the wheelchair motor to turn at 64rpm (around 18-20V should do it), I can use off-the-shelf bicycle sprockets to gear up to exactly that RPM. So the only question then is, is one motor powerful enough to turn all those gears & cogs & idlers, to overcome the friction of 4 leadscrews, and still lift a sensible weight?
I calculate I will need: A large main drive sprocket (will have to make the bushing to fit it to the motor), driving a small cog, which is rigidly attached to another large cog, which in turn supplies drive to small cogs on each leg (so 2 chain rings, and a minimum of 6 small sprockets. To reduce the individual chain lengths, I had wondered about adding an additional large/small pair (so each driver pair drove 2 legs), so that's 3 chain rings & 7 sprockets. Each driver (except for the one on the motor) will need to run in bearings, the top of each leadscrew will also need bearings, so at a rough guess that's 8 bearings so far. Then the chain will require tensioners; 3 options here: Fixed plastic "slide" tensioners ala a car cam chain (which introduces possible wear issues), but these would be very simple to make & fit; plastic guide wheels (more bearings), or additional sprockets (more bearings AND sprockets)... Then there's the chain, of course. Plenty of it, certainly more than on your average bike...
Everything's actually pretty cheap, except for the chain rings (around £12 each) and the lead screw/nuts (but I could potentially make these from bar, which would be cheap). Actually, the lead scews can be had for less than a tenner, but the nuts are ferociously expensive, even from China, so maybe I'll buy the German leadscrews & just make the nuts myself. Of course, one option to save a few pennies is to buy a job lot of broken bicycles, and rob the sprockets off them... but then I end up with a pile of dead bicycles to dispose of into the bargain. Plus, who knows how much wear & rust will be on these things?
Still.... I'm tempted... not ready yet to abandon the idea to a central hydraulic hand-operated jack & latching legs... but I do have a deeper appreciation of why these things cost so much.
Oh - I looked into 2nd hand rise/fall desks - they're certainly available on the 'bay, but the cheapest I saw was £200, and no indication of maximum load. I'd love to find a broken one so I can see what they use for a mechanism...