I took your advice, and dug out my T.4 Pilot's Notes, which read in Part 1, Chapter 10, page 5:
17.
Jettisoning in conjunction with the ejection systems. Provided the CANOPY/SNATCH MASTER switch is on, the canopy is automatically jettisoned when either firing handle on either pilot's seat is operated.
The nav cabin hatch was the same according to the notes.
My guess is the other variants are similiar...I have a B.2 and B(I).12 PN somewhere I will double check with.
**UPDATE:
So, after digging out the B.2 PN, firstly it is somewhat less descriptive on ejecting, but the main difference is it indeed does state the canopy jettison/snatch switch should be pressed first "and then the pilot's seat ejected." It is also stated the navigator must double-fold his table first before following the same procedure (I'd like to hear from some of the former navs on here about that drill). So there does seem to have been a change in notes between the B.2 and the T.4 - I do not have the
PR.3 to verify when this happened. Another point worth noting is that my T.4 notes are 4th edition compared to the B.2 2nd edition I have.
On a somewhat related, but morbid note, in the research I have been doing on Argentine usage of the Canberra, it is possible that the navigator of B.62 serial B-108 may have attempted a 'manual' bale out when shot down on the night of the 13th June 1982. When hit by a Sea Dart fired from HMS Cardiff, the impact struck behind the pressurised bulkhead, somewhere below the No.1 fuel tank. The resultant damage apparently caused the failure of the nav's ejection seat, of which he informed his pilot. Cutting this tragic story short for this post, in the end the pilot was forced to eject alone, and the nav seemingly went in with the aircraft. It has recently come to light however from an eyewitness on the UK side who saw it descending in a flat spin, that it appeared to be trailing something burning in the area of the cockpit. I have since wondered whether the poor nav blew the hatch and attempted to bale out but got caught up somehow. I would be interested to hear from those with flat spin experience whether it would have even been possible to extract yourself from the seat against centrifugal force and do this.