I've received the lens back and now adjusted the infinity focus with the help of an excellent YouTube video -
null - YouTube. from mikeno62. I found that the rubber bush from the drum securing bolt supplied with a washing machine was the perfect size to remove the lens label ring and I was fortunate that mine unscrewed very easily. Following the instructions in the video, I removed the three screws indicated and the front barrel - this exposes the screws holding the focusing ring. there's no need to remove these - just loosen them, with the focus set to infinity, then with the focus ring free, move the focus to something like 20m and tighten them back up. This allows the lens to be refocused to infinity. At this stage, i checked the lens on my KP and it focused on my test subject - a neighbour's chimney at about 100m, with a bit to spare. So, I then put the lens back together - all done!
It was clear from scratches around the screws behind the lens label ring, that someone had been in there before me, presumably to adjust the lens for a non-Pentax body. I'm still not sure why the modified lens apparently worked on a Spotmatic, but not on a DSLR.
Thanks to all for their advice.