Sorry for the late reply, I've only just spotted this thread.
Quote: ...roll of HP5... I sent the film away to a lab for developing, and had some 10 x 8 prints done ...
... results look very grainy to me......
Any chance that your film was over-exposed?
Or, I wonder if the lab used a "universal" developer when developing your film? At least it suggests that the prints were done with a half-decent enlarging lens!
I've not used HP5 that often, but "very grainy" on a 10 x 8 print sounds wrong to me.