Just spoken to the guys at Oxford. There is nothing official on this, and it's certainly not been the subject of a FLM amendment.
Apparently back in the early days of 135 ops(in the UK at any rate) there were a number of incidents of FADEC problems during starts. On observation it was noticed that some pilots were being a bit quick on the start, ie not waiting until all the FADEC captions had cleared from the CAD. It was suggested by Turbomeca to wait "a few" seconds before starting, and I believe that this is when the check Po and T1 requirement was introduced to "slow things down a bit".