It was the switch mode PSU in the actual charger plug causing the problem - the interference was there regardless of whether the satnav was plugged in or not.
If you break them apart, some of them are of a horrendous design and don't include any filters or shielding at all. But then, what can you expect for $1 from China...
The one electrically 'silent' one I came across was a genuine TomTom one.