GuitarJim: thanks for that. From some pokings around before, I came to the conclusion that I should be OK with the 305w from the dimension 5100 - fingers crossed. See e.g.
https://www.tomshardware.co.uk/forum/259116-15-best-300watt-card
- and elsewhere I’ve seen many people say the Dell 305w power supplies have managed OK. It’s got some kind of hideous multi-card input device on the front that someone must have thought business machines need, so I’ll pull the plug on that also.
Thanks again.