Can someone please enlighten me as to what suspension the mk1 Superb has. Info like this has always been easy to find for other cars but seems impossible for the Superb.

I've looked up and seen mentions of double wishbone on the rear of a passat, is this correct?

The double wishbones came in on the B6 passat. The Superb I uses the same single beam suspension as a B5.5 passat.

I'm not sure how similar spring rates etc are, but otherwise most suspension components are interchangeable between a mk1 superb and 5.5 passat.

Rear beam as Passat 5.5, rear is much heavier than Passat due to extra length/weight of the Superb over Passat. So if you choose springs from Passat range, make sure you take the hard ones - either HD or estate.

If you need wishbones/joints, please kindly move towards the front wheels of the Superb, plenty of wishbones and joints there