Hello everyone. I'm the original poster, and thank you all for your thoughts and comments. My apologies for being "out of the loop" but after my original post slipped "under the radar" I hadn't checked back on it since. And as Dawnsh correctly assumed, life is busy for me, so my opportunities to come on board the forum are usually few and far between. I squeeze my family history activities in between my day-to-day job, looking after our two young children, and all the other usual responsibilities that come with having a young family. So, huge thanks to you all for your comments and my apologies for having been absent from the discussion. Please know that this is the result of circumstance rather than rudeness.
I'll try and respond to all the queries that have arisen.
My grandfather's dates and places of service include Kemble, Innsworth, Penarth between 1940 and early 1942, after he joined up and was serving as an Accounts Clerk. From early 1942 to May 1943, he was undergoing wireless training, based at Bishopbriggs and Bolton. Thereafter there was, I understand, a short transit period in Blackpool before he went by troopship to India and on to Burma to serve as a wireless mechanic.
My grandfather was 26 when the war began.
Scouseboy - His trade before the war was 'accountant', which is why he initially served as an accounts clerk. He soon tired of it, though, and wanted something more challenging - hence the training to become a wireless mechanic. He was, I believe, the exception rather than the rule in being accepted onto the demanding wireless training course without any background in radio or a university degree, but he was an intelligent man and very capable. After the war he went back to being an accountant for a while, but within a few years was heading the sales division of a pest control firm! A varied life. And thanks very much for your comments about being proud of my grandfather - I am, very. He was always frank about how he was lucky and "got off lightly" in comparison to many of his comrades in the Far East, but having since done some reading about the role of the RAF in Burma and the role played by wireless mechanics, I now have a better appreciation of the role he would have played.
So, to the photos themselves. Am I correct in thinking that consensus is that the larger, bottom photo is likely to be a photo marking his "intake's" graduation from basic training and that the smaller, top photo is probably one that signals his graduation either from trade training as an accounts clerk or as a WM? With my comparatively little knowledge, I would say that this sounds perfectly probable. My only doubt stems from the rather insalubrious-looking background for the top photo - in front of a row of houses would seem an odd setting for a formal graduation photo (but perhaps not, given the situation and the times?).
With best wishes to all,
Steve