2004 Citroen Xsara Picasso engine cutting out

I've developed a problem on my Picasso after eight years of trouble free motoring in it. Yesterday after letting the diesel run really low I put roughly 30l in and set off out. Whilst waiting outside my nephews the engine suddenly stopped and wouldn't restart. It was turning over fine but wouldn't start. I removed the fuel lines from the filter housing and cranked it again but no fuel was getting through. So after a few minutes thinking I removed the fuel cap and tried again, it started up first time and ran fine. I set of a about a mile down the road it cut out again, so I removed the cap and it fired back up. Now the engine won't rev correctly and it's cutting out intermittently. On my 1.5 mile journey to work today it cut out three times, every time I remove the filler cap it restarts. I'm thinking blocked breather which would explain it cutting out but not the loss of power, any ideas or tips would be greatly appreciated. Thanks in advance guys.