It's amazing what some hub fluid will do. I drained the old fluid (there wasn't much in it and it was pretty dirty) and filled them up (as much as I could put in each as I couldn't find anywhere just how much to put in) and the problem appears to be gone. Hallelujah!

 

To get the proper fill level you turn the hub so the fill plug is at the 2 o'clock position(I think) and the fluid level should be there.

 

Assuming that I've overfilled it - what would happen if I didn't drain the excess fluid as I filled them at the 12 o'clock position (I obviously can do so easily but I'm just curious).

 

the only thing I can think of would be the fluid expanding as it warm up and leaking past the seal.