Glad this was refreshed with madijag's question. Good post to learn from.

 

This is an old thread that I had not noticed before and I once had similar issues with erratic starts on my '94 AJ6 XJS because of low fuel pressure at re-starts, especially with hot re-starts that was tracked to a bad check valve inside the fuel tank. The solution (other than the hideous tank removal) was to install another check valve outside the fuel tank. The valve was installed in line with the hose that feeds the fuel to the fuel rail and it worked awesome. This incident took place back around the time when the initiator of this thread posted his.

Fortunately, this valve had a "T" port on the "trapped pressure" side that initially I just capped. However, later on I installed an electronic fuel gauge on my A-pod and I needed to install its sensor somewhere close to the fuel rail, so the valve's center port was a blessing. I teed a new manifold to the check valve center port and the sensor, plus a new in-engine bay mechanical fuel press. gauge were both attached to this new manifold. See pictures below.

The low fuel pressure problem never re-occurred.

Cheers!