Split hub caps on 90 defender

I have purchased 2 pairs of rear axle hub caps for my K reg defender (200Tdi, rear drum brakes). Easy to fit but split as soon as I get on the road. I have been assured that I have the correct part (FRC4377) but they look slightly different to the ones on the front axle (presumably original). Does any one know if these are the right hub caps OR is there a fault with the axle that is causing the split!

Just in case someone else picks up on this: turns out that the generic hub caps are made from stiff plastic that easily splits with normal movement and metal expansion on a drum brake rear hub; buy land rover's own. these are made from a more rubber like material and can move with the hub.