Hi Darius,
Thanks for your comments, I'll take another look at both circuits again and see what I can figure out.
I should have noted, the 3 video inputs are 0.7V p-p video only, no sync present on any of them.
The output of my 74LS86 XOR gate is giving TTL-level composite sync, with serrations of a kind - they're at line-rate and are actually inverted V-sync pulses as these run all the time from the computer. The video signal is supposed to have around a 0.9V DC offset by the time it arrives at TR5 base, TR6 was then bodged on as an attempt to get the syncs added.
Also, what's on the diagram is partly due to what I've got in my junkbox; I've not been out to buy more components for this yet so some values are a bit of a compromise - also, the output has only ever been connected to a 'scope; I don't count it as 'finished' yet; and it was missing a resistor on the diagram. I've just amended it a bit to reflect what I've actually got hanging off a blank bit of copperclad.
Jeffrey, I know what you mean about split supplies; i usually run my audio circuits off split 15V or 18V supplies; and of course AC coupling doesn't matter much. With this thing I was trying to get it to run off a handy 9V wall-wart; split supplies would make things a bit easier though
Regards, Kat