Thanks for the responses so far. Yes, the HC100 was in dilution "B". I didn't think it was a problem because he'd been developing his film with the same developer and getting good and contrasty results. I will try again with a fresh batch and see.
The light certainly wasn't the best, it was overcast, as most wintry days in London are... I'm still searching for a BW film that can handle that sort of low-contrast, flat lighting reasonably well. It was the first time I tried Ilford FP4+ and I've heard mixed reviews about it. I'd used Delta 100 before in similar lighting and got better results there. Since it was the first time I'd processed film, I thought it might be premature for me to dismiss the film entirely and instead, question my development methods.