Wow...!! Thank you everyone for your advice. The consensus approach seems to be to buy cheap and practice hard. Becky makes a valid point that there are no second chances for the right scene, so buy the best and to hell with the cost. Leon's approach seems to straddle the middle ground insofar as Adox CHS 100 seems to be both good
and cheap.
In the time since posting the question, I've spoken to a friend of mine who is a pro photographer. It turns out he's got a few assorted boxes of out-of-date unwanted 4x5 film in his fridge that he's prepared to let me have for not a lot of wonga. He's also got a Polaroid back that I can use with some of the Fuji and other instant stuff. Therefore, I'll be taking the lot of his hands. I'll probably get a box of FP4+ and some Adox CHS100 for best.
After I become comfortable with the camera by using up the OOD stock, the thought occurs to be that it might be fun to load Adox 100 on one side of the film holder and FP4+ on the other, take identical shots of a few different scenes and see which I prefer the look of. Then, assuming one is an outright winner, it's job done. We'll see
All I've got to do now is get someone to sew some curtain blackout material over the top of one of my old tee-shirts and I've got a ready-made "snood" for the Wista as well. I already have a cheapo (but workable) 8x loupe to aid focussing, so I should - hopefully - be set up by the end of this month.
Terry makes a good point regarding speed of use but, quality aside, slowing down and getting the shots bang-on in-camera, is most of the reason I'm turning my attention to LF. I doubt it'll ever "replace" my 35mm or MF shooting but I get the impression that LF work can only be done without compromise. That seems like a good discipline to me