Sorry for the resurrection, but what a fabulous thread.
25 years ago I started a book on the Gannet, unfortunately it still remains a manuscript.
The pilots underwater escape seat system came about in the mid 1960s, and was fitted to remaining AEW.3s from the late '60s on.
I'm trying to remember the manufacture of the device, as they released a brochure on it called 'puffing them out' or similar, it was someone like Dowty.
An easy way to tell if the system was fitted was the two 'ejection seat' style warning triangles under the canopy, and what looks like an extra window just aft of the sliding hood. This latter feature was an implosion panel, which broke through at a pre-determined water pressure to flood the cockpit and equalise pressure before jettisoning the canopy and initiating the seat release - all by the magic of hydrostatics and compressed air.
Interesting to read the bit about ECM.6 XG832 accident, this was very local to where I came from and was still a fabled tail then. It was undertaking circuits at St Mawgan before ending up in Melba Pit, in the Clay works area.
I knew someone who saw the wreck the next day, and think he found a part of it which he had when I knew him.
Would love to hear first hand accounts and stories of the Gannet!