Thanks for the links. I will definitely have a look.
My friend uses the HC110 developer regularly, and I think it was quite a "fresh" mix. I think he reuses it 4-5 times before dumping it, and anyway he developed another roll after I did mine, with much better results. It is making me neurotic. I do bang the tank hard, but I guess possibly not hard enough.
It's true, I shouldn't have made the comparison between Scala and FP4. It was just convenient because the photos were on the same page - but there are also glaring differences between the FP4 pictures and Delta 100. It is also possible that developing issues aside, I just don't like the characteristics of FP4, and that's a subjective thing. I tend to favour the tonal range of films like Neopan Acros, Agfa APX and Scala etc anyway.
Thanks for the thoughtful responses.