Remove the prop shaft, put the handbrake on so the rear wheels can't rotate & undo the big nut in the centre of the prop flange on the front of the axle diff casing. It might need a sharp crack with a mallet on the end of your socket bar to shock it loose -yeah, I know you shouldn't hit sockets. They don't call me Manky for nothing.
There might be a tab washer behind the nut to stop it vibrating loose. If so, bend the tabs back, clear of the nut first. Tap the flange off, (it's on splines), & you'll see the oil seal behind it.
It's a very common leakage point on Reliants. Unfortunately, because the flange rotates, it only takes a small amount of oil to spray the stuff everywhere. With the flange out of the way you should be able to prise the old seal out. It might have a pressed tin casing around it, not sure. If not, it's just rubber. Try not to damage the aluminium diff housing when you hook the old seal out. If you do scrape it, smear a little gasket sealant around it before tapping the new seal back in. I usually use a large socket, placed open end over the seal, to knock it back in evenly. Then slide the flange back into the centre of the seal, lubricating it with a sear of grease. Refit the nut & bend the tab washer up to secure it if it has one.
Inside the diff casing is a crushable collar. When the flange is first tightened down to the correct torque settng, it should squish down to take out any play in the gears. I once replaced the collar when changing a seal but haven't bothered since & it didn't seem to make any difference.
Torque setting for the nut is 70 - 85lbs/ft.
