Yes, after I posted there I checked again and realised that the more powerful ones were probably standard sized. I've found a couple of sites with micro-sized ones with higher wattages, but they're all a long way away.

And the airflow point is worth thinking about too. There might be a reason they left a fair bit of space round the unit they used. (Or it might just be that that was the size they came in.)

I've found a place in a nearby town where they have what might be a suitable one. It's the right sort of size, but that's not the only consideration - but the bloke on the phone didn't know too much about computers, which isn't too encouraging in a computer store...I should find out tomorrow.