Way back in 2011 when I built my 150 Sigma I had the same issue on initial startup. Caterham fixed it at the PBC by balancing the throttle bodies and then setting the TPS to 1.04. They assured me that was all they did! I now do this every year!!
Sorry you are having that problem.
Hopefully be collected next week, been sitting there for ages since September
Be fascinated to know what it is
Well, if anyone can sort it, Eric can. Looking forward to hearing his diagnosis.