Just back to say that I've had another roll developed from the same batch - at a different lab - and all is well.
Not sure I'm completely sold on the high contrast of this film compared to HP5, but that's another story. Probably just not used to it, I tend to do all the contrast adjustments post-process in Photoshop, so maybe I just need to adjust my workflow with this film. I've kind of got used to HP5 being 'flatter', which allows me to start from a more 'even' base, I suppose.
Thanks again for all the comments above.