Vauxhall Corsa corsa won't start

My Problems began with my key turning over and the engine dying. I would need to rev the car for a good ten minutes to get it started. I needed a new clutch, gearbox, coilheads, sparkplugs and handbrake cable and part of the exhaust all of which were fitted. Now the car won't start at all and the engine dies quite rapidly once the key is turned. I tried jump leads but they made no difference. I had a mechanic check the reason for not it starting and he did a diagnostic check, a wiring check and stated the fuel pressure was ok. He suggested a new key "may" resolve the issue as the chip may have gone it in. I went to the key shop who did a test on the key and stated the chip was fine. Would anybody have any idea what might be wrong? Many thanks