FFS!
It's almost definitely the hub oil seal which means either the seal's fookd or possibly a bearing's gone and pushed it out - you need to take off the wheel, brake caliper (put the screws back in pretty quick else you'll lose any remaining hub oil), hub cap, half shaft, locking nuts, hub and bearings and replace the seal (and any necessary bearings which you might as well replace anyway as they're only a coupla quid each) - first time 2-3 hours work, after that it's about half an hour.
You can buy the whole kit from Tamar 4x4 (2 bearings, seal, locking nut) for £8. (well, you could about 2 months ago!)