Purely because I simply wanted one, I've eventually picked up a purpose built NAVTEX receiver. This is a Target NAVTEX Pro Plus unit from Nasa Marine. Second hand, and faulty, of course!
This is an older model, with external channel switching, as the people at Nasa Marine have been kind enough to supply me with a hand drawn copy of the receiver circuit diagram proving this. So the receiver itself can only handle the 518kHz broadcasts. To cope with the 490kHz National services, the active E-field probe antenna has frequency changing capability.
The receiver works - I have it connected to a short indoor length of wire, which allows it to receive some of the strongest signals. Its internal back-up battery is not in a very good way, but that's not much of a worry.
The antenna and Local/International switch box have issues. There is some corrosion on the antenna PCB - one tantalum bead capacitor has lost a leg - but it doesn't look too bad. There is a PIC micro-controller that looks to be serving as local oscillator, which has a lot of green gunk around and under it. One of the PCB traces looks rather poor as well. So long as the corrosion has not got into the transformers, repair shouldn't be too tricky. There is a destroyed track in the switch box.
So, a bit of linking out and cleaning up required!