How much would it cost to replace the back cover on a rear diff?

Banny

 

IMHO hour to remove, 5 mins to replace cover 45 to put back in max.
Pros like API faster.

Gasket + fresh oil to, add plus vat of course.

Sound £100 to me?

 

Very easy job and you can do it yourself.
Drain the oil so you can re-use it.
It will be easier if the car is on a jack but it can be done with the car sitting on the drive.
If you are careful the gaket will be OK to re-use.
Cover is held on by 8 ???? bolts and easy to remove.
To put the oil back in the diff, use a washing up liquid bottle and a length of rubber pipe.
If cover is a little stuborn to come off just hit it sharply with a hammer.

 

With Dazza, very easy DIY, although I wouldalways use new oil as its cheap enough, use a pair of ramps and you can then use the trolley jack to help you put the diff in place!

Simon