Generating PAL computer signal using a PIC

I'm trying to generate PAL tv signals using a PIC24FJ64GB002 (datasheet:

http://www.kynix.com/uploadfiles/pdf8798/PIC24FJ64GB002-I2fSO_94644.pdf)using a similar method to the circuit described half way down this page. The only difference is that my circuit needs to run at 3v instead of 5v due to using a PIC24 so I've recalculated the necessary resistor values and the closest ones I can find seem to be 220 and 470 ohms.

I'm connecting to the TV input via a scart lead connecting the video signal to pins 17 and 20 on the diagram on this page.

But I get no picture at all on the TV. It might as well not be connected.

I'm thinking one of three things could be wrong :-

1. My timing is out. I've posted the diagram at the end of this post showing the timing on the two pins for my test and as far as I can tell I'm generating the correct sync timing and the other signal is just some stripes for testing. How accurate does the timing need to be? I'm using the timer interrupt on the PIC to start each line 64uS apart.
2. I'm using the wrong resistors to mix the signal. Can someone check if I calculated the correct values to use at 3volts? How accurate do the voltage levels need to be? I chose the closest standard values, is this accurate enough?
3. I'm connecting to the wrong inputs on my scart lead? Everything I've read shows this as being correct though...

I am generating what I believe to be the correct frame sync pulses too. Although again, everything I've read seems to indicate that even if I get this wrong I should see something and as every line is the same loss of vertical sync shouldn't matter for this test?

Anyway, any input and suggestions would be welcome.

Additional question

If I want to use RGB signals on a scart it looks like I need to have 3 connections for R, G, B and one for the sync on the composite video pin. And then each one has it's own separate GND connection... Is there any reason I can't just connect those 4 together?