As a final resort, I stripped this down until I could access the I/O board. I wiggled each of the connectors. It has two old-fashioned 8 pin DIL chips on it. When I pushed them down they gave that distinctive "click" of re-seating. Rebuilt it (only one spare screw left over) and now it works (at least as a USB device, havn't checked the network connection yet).
Result!
But a Brother or HP next time, I think. Thanks all for the advice.