Okay, I've discovered that an old Magellan Nav 6000 will do the trick. Does anyone have one that they want to sell, assuming that it is in good working order?
I have one, but I wasn't meaning to sell it: it's my "spare" plotter. Things you might want to know about it before buying:
1. the processor is decidedly slower than more modern units, and you spend a lot of time waiting for screen refreshes; this gets worse if a big chart is loaded.
2. it's monochrome, which may or may not be an issue.
3. initial acquisition of GPS data can take many minutes.
4. if you don't use a 12V supply it goes through a set of 6 AA batteries in less than 20 hours.
5. It is technically handheld, but it's pretty big and heavy and you'd probably not want to hold it for very long. I used a mounting bracket.
If you don't mind monochrome, and will be using a 12V supply, Gael Force currently has a fantastic deal on the smallest Navman unit which IMHO is probably a much better bet.