Well, I think I may have solved the problem in a sideways sort of way and a little help from my friends.
The power resistors arrived this morning, 3 watt wirewound, 2.3 ohm, 2.7 ohm and 3.9 ohm. No single one did the trick but putting two in series did make quite a difference.
However, in the meantime Dave Milbourn of ACTion suggested that I look again at the mini cassette motors he had sent me as he had found them to be OK in this sort of situation. Although the shafts seemed to be free running, the problem turned out to be the couplings. They are nominally supposed to be 2mm but are nearer 2.3mm I think, although perhaps not actually 2.3mm as they won't fit on some of my 540 siize motors. Also the propshafts themselves are slightly under 2mm and the combiination was introducing sufficient misalignment to seriously inconvenience the mini casssette motors.
I have removed the couplings and substituted some flexible insulation tubing and both motors now run sweetly with sufficient power to move the hull along quite smartly just as Dave promised, so the sun is shining once more (actually it's snowing again
)
One shaft is still slightly slower than the other but a bit of fiddling about with the motor mount should hopefully fix that although it's not serious.
A couple of pics of the current setup below, one with the Action ESCs, Mixer and the Spektrum RX.
Colin