Hats off to Ilford. About an hour after sending them my question, they revert with a nice reply. I have taken the liberty of quoting it here, I hope Ilford doesn't mind:
Potentially, your film find sounds really interesting! But I'm pleased too, to see that you also acknowledge that it can be sometimes disappointing when such old films are processed ie old films can have high fog, or mottle and will inevitably have lost speed and contrast.
That said, any dev times I've typically ever given out to other people with film finds as you of obsolete/old film products - have actually led to surprisingly good results. So hopefully you too will be pleasantly surprised, and have some interesting images to try and identify a time period/faces/buildings etc! I wish you luck.
I would advise that you use D76 stock 20c - and the time I would advise you give is 12 minutes.
What you might wish to do - is simply process a few frames (clip test) at that suggested dev time, and then tweak it accordingly for the rest of the film - if you need to.
Just as a final bit of info - in case its of interest to you! HP3 was launched around 1941 - and discontinued in 1960 (when it was superseded by HP4).
So maybe that can help in your age identification - if your images are successful!
(a sidenote - They suggest D76, since I told them I had D76, Xtol and Rodinal at hand).