OK, it's retraction time . . .
Earlier I said I didn't think the "GrimReaper" was associated with the Grapple series of nuke tests, well -
w r o n g!
I've enlarged part of John's pic and you can just make out a mushroom cloud in the centre.
Then, knowing John's propensity to take thousands of photos, had another shufti through his site and discovered another couple of pics of the "Grim Reaper" Canberras. In one shot, you can make out a serial - WH887 - on the B.2 (the other Can in the pic is a T.4). WH887 was with 1323 Flt which became 542 Sqn. This flight/squadron was formed initially to do "sniffing" for the Grapple tests so it's fairly safe to say that John photographed one of the Oz based 542 Sqn "C" Flight aircraft en route to/from the UK. (887 went back to RAF Upwood where 542 Sqn were for a short period before becoming 21 Sqn at that base.)
Therefore, the "Grim Reapers" Canberras WERE part of the nuke tests (so Gainesy was right!
) and the nose-art was obviously a squadron thing.