It all sounds very curious! By 'air test' I guess you mean a maintenance check flight? You refer to 'we' but with no pax, so presumably accompanied by another pilot as crew (it's obviously a twin engined aircraft) - was it they that reported you, or somebody on the ground, or did the flight not end well and the AAIB get involved? 'Mr investigator' sounds more like AAIB than police. Are the 40 KIAS and 10 kts g/s recorded data, or pilot observation? What does "minimum safe speed for single engine flight" mean - it has no regulatory definition. If you are trying to prove a factual statement, to refute a claim, you should relate the IAS to an RFM performance standard - Vtoss, Vy, OEI HOGE etc.
I don't expect answers, but these are the kind of thoughts that jump to mind when reading the account.