Won't start after replacing valve cover gaskets

Just replaced the valve cover gaskets on my murtaya kit car which is running an ej20k sti v4 engine and simtek ECU.

The car has been off the road for a couple of years but was back up and running fine last week, other than the leaking valve covers.

After replacing the valve covers it will sart for a few seconds and then die. I've re-checked everything I removed, fmic pipes, valve cover breathers, ht leads, plugs etc, but still no joy. Also hooked up a wide band that I had lying around from another project. It initially runs at about 15 AFR and then leans out to about 17.5 and stalls. 

I've only really worked on rover V8s before so new to turbo engines. I'm thinking the next step would be to check the fuel pressure and vacuum, just wanted to check I haven't missed anything obvious first.

Cheers, nick