A quick update.
I found the eBay unit didn't fit the connector on my car - the security cutouts were different. So I took it apart and checked the circuit, which looked broadly similar to my old unit. Then I cut out the security cutouts so it physically fitted the connector.
Plugged it in. No joy.
I resoldered as much of mine as I could - it was difficult to get enough heat into some of the joints, even though I have a 60W soldering iron - may need to break out the big guns 120W iron
Anyway, that didn't work either.
Then as I was messing with things, the blower suddenly sprang into life! It turns out that there appears to be a faulty connection going in to the next "box" along on top of the blower. There's a plastic cover over it. I have no idea what's under there, but if I wobble the wires going into it I can make the blower start/stop working, all with my original transistor unit.
However, another problem has since emerged. The 8mm headed bolts that hold the plastic cover won't undo. Whatever's holding the captive nuts underneath is just turning. Argh! The rearward one is totally inaccessible as it's under the front scuttle. I feel like I'm being punished now!
I'm wondering if it's possible to undo without removing the blower unit. I don't want to as there's lots that has to come off and I'm not even sure if I can without degassing the AC. Double argh...