Does the fact that the Gnat was able to return to its home base mean that the AAIB don't feel it's a mechanical issue with the type of aircraft?
No...it means nothing.
The AAIB is not a regulator, and if you think of all the civil aircraft accidents that take place, how many of them result in a grounding in the immediate aftermath? Those that do are either self imposed by the operators, or by the regulator following a series of similar incidents/accidents.