CROSSPOSTED AS I KIND OF ACCIDENTALLY HAVE TWO THREADS RUNNING (one mentions widescreen but this is probably the more recent one)
OK, I may well have cracked it.
I encoded my 4min test movie in sorenson in three ways. It seems that the previous problems were to do with having too many P frames, as when I returned to the 3 P frames setting (not as good quality as the 1 P frames setting as this doesn't use B frames) it worked.
I also switched the audio in sorenson to PCM and in MyDVD took 'fittodisc' off and changed the audio to LPCM, not sure if this had an effect, but as this is the first time I've got the thing to work it'll be how I do it from now on!
Also, there is a strange thing. MyDVD menus seem to be in 60HzPAL. Before anyone says PAL is 50Hz, there is a setting most TVs (in the UK at least) can deal with which is for watching NTSC videos on PAL equipment, most TVs therefore wouldn't reveal this, but when I put this through broadcast kit with a frame sync, it shows up that the video rolls. According to our format guru here at the BBC, this looks to be an NTSC menu which the domestic DVD player outputs as PAL at 60Hz...
anyway, I think I can live with that as domestic TVs will handle it fine (on the most part!)
anyway, if I discover anything new I'll keep posting!