Many thanks for everyone who took the time and trouble to reply. I've gathered a lot of information as a result!
For the time being I'm going to chuck the plans for a cascade washer and adopt the following washing regime:
Dip / soak in a bath of fresh water for 30 secs to 1 min
Tray washer for 30 mins
Test at least one work print from each session with a residual hypo test kit.
If washing starts to slow things down too much I'll build some of Les's tray washers and run them in parallel to my existing unit.
Then all I have to do is dry the swines flat...!