I believe a problem is solved

After trying a long list of possible solutions to the rev limiter problem, none of which worked I finally checked the wiring again. The diff speed sensor definitely goes to the dash connector 3, that's why I get rpm and mph displayed in the instrument cluster and it also goes to pin 14 on the loom side of  the X20 connector. But when I took the rubber cover off  the engine side of the connector pin 14 wasn't connected at all, there wasn't even a wire crimp inserted in the hole. This meant there was no speed signal going to the ECU.

A number of people including Dave, John and Karim have suggested that it requires a speed signal from the ABS to the ECU on pin 9. As the Legend has no ABS sensors one could try and connect the diff sensor to pin 9 of the X20 to fool it into thinking there was an ABS signal.

To cut a long story short, that's what I did.

As a quick fix and try out I simply cut the wire (black/slate I think) which went to the X20 connector pin 14 and spliced it to the wire going to pin 9 instead.

Quick fix to save me having to remove  the scuttle - winter job!

X20 Connections
9 - BR/RT-WS/GR/GE (WS/GN) ABS SIGNAL DME-ABS / OIL TEMP>INST M3
14 - SW/WS DME<SPEEDO OUTPUT (M1.7.3 MSS50 M41)

The two test runs I've done so far I've run up to 6,500 many times without any problems at all. I'll continue to verify this over the next few days but it does look good! 

Another thing is the ODB II interface is now reporting mph which it didn't before, although the value is actually incorrect as the pulses from the diff sensor are fewer than from an ABS sensor (ODB shows 10mph when it should have shown about 40). Also the diagnostic error "No speed signal" has disapeared!

Now she fly's !!!