Thanks for all the great tips everyone.
I went with Ilford Delta 3200 in the Pentax MX, with the Pentax SMC 50mm f1.4...
I also dug out a Miranda MS2 Super (Pentax SMC 50mm f1,7), and put some expired Kodak 400TX in there.
I managed to stick to natural lighting only, so there's a fair few that didn't come out well, but I've got some cracking photos back from the lab and somehow my boy Cosmo is still with us so I have a bit more time to try other films!
Don't know what order the pictures will appear, but the the one where he's looking back to the camera is Kodak 400TX, and the one where he's looking down is the Ilford Delta 3200 (shot at 1600).
As ever, tips, feedback and advice is welcome!
Thank you all for helping me get some pictures I'll cherish for the rest of my days.