Ignition/starting problem
Thanks for the replies.
It seems most likely it's a problem with the brown wire circuit - the car starts instantly using the button and runs great.
To bypass the ballast resistor I connected the two terminals that normally go to the ballast resistor to each other.
I realize using the button changes the way the system works - my point was that it couldn't be causing the problem when using the key as none of the original wiring had been removed.
I think I'm going to check the brown wire circuit and if there's not an obvious issue just fit a permanent starter button so I only need to use the IGN1 circuit.