Well, after a month of investigation, the problem seems to have been the T4.5 thermo couple probes...
The engineers looked at the master control box, starter relays, the starter, electro start valves, FCU, lubrication pump, fuel valves, injectors, etc..
The problem was eventually found to be one or more of the T4 probes sending false temperature readings to the VEMD. During the start, despite apparently taking T4 up to 800 oC (and higher on occasion, for a few seconds), it was actually much cooler and it was this that caused the stagnation, usually at around 36% Ng.
A new T4.5 harness (obviously, you need to buy all 4 new probes...) and she's now back to form.
Just a heads up, in case other EC120s show similar symptoms - you might save yourself a few $000s on fruitless investigation. This intermittent problem originally affected only 1 in 10 starts, but it got progressively worse until it was happening 7 times out of 10 and usually only when cold. Thanks for all the suggestions
