Worth checking on Ebay. We got a hardly used (and very clean) Thetford C400 with cassette for about £100 a year ago for our trip boat. The guy selling them gets them, I think, from caravans which are being broken up. At that price, I reckonned that it would be worth any cleaning I had to do I donned the Marigolds, put the unit in the bath and used the shower head. I think we downloaded installation instructions from the Thetford web site. These Thetfords now come so that the cassette can emerge from either the right or left but the old ones , I underdstand, had to have a cassette of the correct orientation.
Haggis