Ok, so I perhaps naively thought I was the first person to ever think of this - go figure.
Anyhow I managed to get my hands on a couple of cordless drillers which have an expired battery and thought that if I added a power jack (socket) to one of the drillers I could buy a cheap external "wall wart" power supply to drive it. It turns out that it is not that easy to find such a power supply but I have recently bought a bench power supply which can deliver up to 30V and 5Amps so I thought I could use that given that the driller runs on 18V and draws between 1 Amp and 3.4Amps in operation (the latter when I held the chuck to simulate resistance (and torque current)).
The snag is that some guy at the local electronics shop told me that if I use the bench power supply I would almost certainly damage it (specifically he said that switched mode supplies should NOT be used to drive a driller and that I should use an old style "brick" transformer).
I have in fact already operated a similar driller using the bench supply (to test motors for my robot) but this was only for a few seconds at a time. There wasn't any evident harm to either motor or supply.
But was he right? I obviously would not like to think I had saved a few Euros only to have to throw away my new bench power supply.