Further info. Done some digging with nvidia-settings --query
and it appears the refresh rate is wrong
nvidia-settings -q RefreshRate
Attribute 'RefreshRate' (mythtv:0.0, display device: TV-0): 5996.
'RefreshRate' is an integer attribute.
'RefreshRate' is a read-only attribute.
'RefreshRate' is display device specific.
'RefreshRate' can use the following target types: X Screen, GPU
My XF86Config-4 contains
Option "TVStandard" "HD576i"
Option "ConnectedMonitor" "TV"
Option "TVOutFormat" "COMPOSITE"
Option "NoLogo" "1"
so it appears that even though I specify a PAL output i'm getting an NTSC refresh rate.