Yes there is a cure which usually works. If you have replaced the oil seal and are still having a problem you have to dismantle again and fit a new oil seal but without positioning it as far into the housing as usual.
i.e. if you use some sort of spacers (cardboard will do) to position the seal about 1/16" away from its normal position it will run on a new piece of the axle and should be as good as new. The spacers are to ensure that it sit square.
Thanks for the replies. I stripped the axle down tonight and found that the oil seal inside the hub wasn't leaking, I had already positioned this in about 1/16" so the seal was on new metal. The leak was coming solely from the flange joint.
On reassembly I offered the halfshaft to the axle (without gaskets and 'O' ring) and found something I wasn't aware of which is that the inside face of the half shaft does not actually touch the hub. There is a gap of about 10-15 thou. The gasket I had taken out was thinner than this leaving the 'O' ring to do all the sealing which it had failed to do.
On reassembly proper, I used a thick paper gasket and also the thinner grey type (for good measure) and I am hopeful this has cured it. A 5 mile trip didn't show any signs of the leak re-emerging but time will tell!
Doesn't the main bearing sit slightly proud from the hub face by a few thou? I guess you need the thicker paper to seal the resulting gap twixt hub and halfshaft flange.
I found the the o ring held my hub and halfshaft apart. Once all screwed down and the wheel on I guess it closes up.
The correct gasket for the hub to half shaft joint is quite thick which ensures that the outer of the hub race is securely clamped to prevent it spinning in its housing.