Thanks for the answers so far. I'm particularly interested in the regulator/whistle connection (we thought the problem was the power supply lead to the intercom box - not the actual power supply). So thought problem solved (delayed flight for an hour to do this) before departing...
Understand about the 'blunt tool' leaning method - but was trying everything! Whilst there is a paucity of both instrumentation and controls, it does have 2x EGT and a CHT indicator - so I was happy that all the temps stayed okay throughout.
Will post back once the tech gets his hands on it!