Well, today my search finally paid off. For Simon Lack, at least.
In doing a Google image search, I found a Spotlight Casting Directory page that had a very young Simon Lack on it, circa 1939. (He'd be 24 if born in 1915, which most sources say.) (The photo was newly uploaded as a memorabilia company in London was offering it for sale.)
It listed his height - 5' 9" I have to admit I was surprised by that. He'd always seemed much shorter than that to me. I guess everyone he ever acted against must have been very tall. (Patrick McGoohan was 6'2, so he was 5 inches shorter than him...I guess that's why he looked so short in his Secret Agent ep when he was speaking with McGoohan.)
Anyway, the paper also listed three movies he'd been in...one of which was They Drive By Night (1938). I checked the IMDB and he wasn't listed in it. Then I checked YouTube, and glory be it was there, uploaded in its entirety. )Being 74 years old I guess it's in the public domain.
I started watching it, and a very young Simon Lack appeared within the first 5 minutes. As the star of the film is coming out of prison, Simon stops him. He wonders if Shorty had known Allen in prison. Shorty give shim a flip reply and walks past, until Simon says that Allen is his brother. Allen is due to die at 9 o'clock am for having killed a woman. Simon speaks in a soft, Scottish-accented voice (which of course makes sense as he is Scottish!) and then bows his head as 9 am tolls and inside the prison he knows his brother is being hung.
Simon Lack on the left talks to Emlyn Williams as Shorty on the right.
"Roy Allen" wants to know if Shorty knew Allen in prison (they never give his first name) and how he took his last day before his scheduled execution.
A bell begins to toll nine am. (That white streak at Emlyn's mouth is a cigarette.)