Hello Charles
Thanks I did have a good journey back from Levington. I think I chose a good day (Cup Final Day), there was very little traffic and no hold-ups at all, did the 320 miles in just over 6 hours including a lunch stop. Still knackered at the end of it though.
Re the ST40. You are correct the ST1000+ does support the NMEA protocol. However the NASA unit outputs NMEA 0183 sentence HDG. Unfortunately the ST1000+ does not accept that particular data (Heading). It seems that Raymarine have deliberately done this to force you to use "Seatalk" which is Raymarine's own proprietary system. It is possible to get "cheap" external fluxgate compasses but I do not think they can be (easily) interfaced with the St1000+.
The ST40 display unit acts as the interface and converts the signals from the (Raymarine) external fluxgate into "Seatalk" format which the St1000+ understands.
I chose the ST40 because it was the cheapest option. The display and compass came as a package. I could not find a lower cost alternative at the time.
According the the handbook, using an external compass should automatically disable the internal one fitted to the tiller pilot. It did not in my case, I had to physically unplug the ribbon cable from the circuit board. It worked fine then. Does not inspire confidence in the product though. Another issue I found was that my ST1000+ connecting plug was wired differently to the information given in the handbook, the Seatalk wire was connected to a different pin, easily corrected once I had diagnosed the problem. Annoying though.
Hope this helps.