Don't follow Lilley's website prices, phone them up, Schluter are very strict about how people advertise prices on the net, i'm certain you'd have got it cheaper.
As for the Anhydrite screed situation with Ditra, you can't win.
Gypsum adhesives stick to gypsum, but not well to Porcelain or Ditra. Cement based adhesive stick well to concrete, Ditra and Porcelain, but its a huge risk following what Mapei and Bal say.
If it goes wrong what do they pay for, the cost of the adhesive, or the cost of moving furniture, pulling up tiles, cleaning the floor, then retiling the whole floor again, unlikely.
Not worth the risk, and it is a big risk to take.