only after consultation with engineers and in view of the fact that there was, at first, no apparent damage
I suggest you use a different engineer for advice - were they Rotax Qualified? The Rotax is very different from the standard Continental type.
It was always flown within gliding distance of an airfield, which is fairly easy to achieve with a motor glider.
EFATO?
Amongst those engineeers whom we consulted there seemed to be a general view that the short flight with a diesel/avgas mix would probably have done no harm.
see above
I mistakenly filled my 20 lt jerry can by walking up to a 97 Octane pump but foolishly using the nozzle from the immediately adjoining diesel pump. Check and check again, as they say.
The smell didn't give you a clue?