1988 tdi 300 disco rear axle oil leak

[Black VXR]

Looks like I have a rear diff seal gone. Have checked the breather and it was blocked and but dispite cleaning the pipe out with an air line and making sure the breather is working it still loosing oil. Its in little spots all over the rear door and the spare wheel cover. Is this easy to fix or a garage job and how much are the seals please?

Just been on a 50 mile run and now there is a little puddle under it...

Thanks