!UPDATE!
After 4 months of relatively consistent use, the front wheel bearing started to make ugly noises again. On disassembly there was no sign of the butyl rubber seals that I had made and fitted (see above posts). They seemed to have become completely chewed and then dissolved into the grease, which had turned black. On the plus side there was no real sign of corrosion in comparison to that seen in the past without seals. The fact that the cones were still badly pitted suggests that the materials used are not great. Wheel bearings should last longer than the approx 1400 miles they had covered.
Not to be put off, and in the name of science
I have tried new seals, this time cut from a sheet of silicone rubber, which should (might) be more resistant to attack.
More results as they come,
Cheers,
Ed