Thanks for the reply. I don't seem to have that option (the software version is 5.08.6), but there is a similar "samples per scope trace" under file | setup | scope. I have tried increasing it to 1,000,000 (the maximum), which works at 1ms/div (I suppose about 20ns/sample) and just about catches the sync at 10ms/div.
I suppose this means the normal scope mode software isn't suitable for the unmodified sync signal. Would it be worth trying the "save to disc" trigger option?