I noticed that SJS have some MkI frames going for a song (£125 against £499 for a MkII frame).
The £125 is for a frame only. The £499 is frame, fork (£99) and headset (£22ish). If you're only saving £150 it's not worth it unless you can make savings elsewhere.
Can anyone tell me the difference in practical terms between the two framesets?
I can't with any certainty. I bought one of the first shipment of MK2 Ravens, though as I was getting the pennies together it was the MK1 I was looking at. I know the differences between the Tour and the Sport were narrower on the MK1s, but I don't know if all the changes were to the Sport or if the Tour also changed. One material difference is the chainstays used to be Reynolds and are now Thorns tubing, though I doubt that's a practical difference.
The MK1 was also available in fewer sizes, had more colour choice (some of it fragile paint) garish graphics and only black forks. The earliest two reviews on the Thorn website (Cycling + and Cycle) are for the MK1.
If you see one you like in your size, why not ring Thorn and get a quote for having it built up into a complete bike? If they'll do that and all else is equal you should still get a decent saving.