I bought an Airkey wireless keyboard from eBay; £11.50, IR, presents as PS/2, range of up to 30ft. Seems to do the job; the trackpad thingy isn't perfect, but good enough for basic X use which is all I want.
Surprisingly no one seemed to comment on my query about RF wireless keyboard security. It seems like the sort of thing someone like Adam Laurie would have had a look at, but I couldn't find anything obvious. Anyone got any pointers to research about these?
Oh, and tonight I had to fight interrupt conflicts to make the IR PS/2 mouse work - that machine hadn't had a mouse before and it turned out several things were sharing IRQ 12, leading to Linux not even finding the port. So I rejigged things around in the BIOS so it was saved for just the mouse port and that managed to make things work. SiS 630E chipset for anyone else seeing a similar issue. It felt like I was back in the 90s.