Thanks guys for the replies. Initally I did think it screamed of timing issues... but now I'm really quite sure the timing is correct. I've had valve covers off, the dist cover off and the plugs out, and by pulsing the ignition on slowly I can follow the whole engine working... so for all cylinders, in this order... I do get intake valve opening and cylinder moving down, intake closing and the compression stroke, a spark for the corresponding plug, then the piston moving down and exhaust valves opening. The dist and valves all follow the firing order.

My main concern is why fuel only seems to be reaching cylinders 2 and 4. The other plugs don't have a whiff of petrol on them whereas 2 and 4 definitely do.

(In my original post I should've said that the pouring out of petrol from holes 2 and 4 is only after pumping the throttle and so flooding it)