I forgot to mention, British Encoder (www.encoder.co.uk) are a UK manufacturer/supplier, and are helpful. Not the cheapest option, but a direct drop in replacement for my Denford Cyclone was about £200.
Zapp Automation also supply some encoders, and Gary (who occasionally frequents here) is very helpful.

Other option is US Digital, where you can pick disc/reader head options to suit your application.