HI Neil,

Any of the MGB parts suppliers will have these:

Moss Europe
Brown and Gammons
MGB Hive

You just need to work out what type of rear axle you have - its probably the later tube type - but have a look an compare it to the pictures on the Moss Europe web site:

http://www.moss-europe.co.uk/shop-by-model/mg/mgb-c-v8/clutch-gearbox-axle/axles-differentials

Seals are only a couple of pounds. When you change it also check the diff oil level is not too high and make sure the axle breather is clear.

Cheers
Jeremy

Many thanks Jeremy