Further update:
Decided to walk before I try to run and got the UM1-DE working with PLA running through both extruders. The first print went well, although I'll need to tweak the feed settings (or something, as it wasn't pumping out quite enough PLA to keep up with the head movement) for the second (new) extruder. The first (old) head worked perfectly though.
I'm going to do a bit of PLA printing with different colours, etc, before I try rushing on to PVA tests. Still looking forward to them though.
Does anyone know if it's possible to buy the old bowden tube anywhere, or its specs so I can buy a generic piece?
Cheers!