Look for a little groove worn into the axle itself done by the seal over the years. This may be why it is leaking.
If the seal has an internal spring that can pop out, not familiar with the seal sorry, but if it does, remove it, find the join and undo it. Snip a few mm off the female end, rejoin and refit. This will tighten up the seal a bit and hopefully take up that little bit that you need.
Thanks Ash, will have a look.
I don't think they have a spring, but I'll have a look anyway..
A few weeks ago I noticed that it appeared to be leaking again, but it wasn't flowing, as it's not terribly difficult to take the axle out and swap the seal over I'm going to do that anyway, but I may have just not cleaned it up properly after the last time I did it..
EDITJust checked, there is a spring inside the seal, thanks again for the tip!