If experts did ascertain it was arson, at least the suspects were not rounded down to two. As there would be no call from Nevill to Bamber.
Bamber would be a suspect, but he could claim it could have been anyone.
But a bit weird, an arsonist selecting a remote farm at 3am.
Getting a fire going in such a big house that was powerful enough to get upstairs into 3 different bedrooms & kill everyone in their beds would be very suspiscious. Sleeping tablets would not be strong enough.