It is a fiddle to change the seal, but I managed it so pretty well anyone can.
Remove the drop arm, clean everything up, remove the dust cover if there is one and then try and get the circlip out.
Big tray/loads of old papers underneath, start engine and turn lock to lock, this will push out the old seals.
Fit new GOOD quality seals, I used a piece of plastic drain tube as a drift to get new seals in place, and that's about it.
I did this 3 or 4 years ago and no leaks since. Of course the box itself is now tired and needs replacing, but I gained a few years on it, so I don't mind that.