Thank you again - and to help anyone else, I bought a £4.60 delivered
Laser 1575 Hex Drain Plug Key
which is 17mm one end, and 14mm the other.
It is well-made, to the point that no dings/marks have appeared from me using it to remove both gearbox plugs, and it only fits halfway-in, if you see what I mean, and the plugs were in tight.