original broadcast format?

I would guess that as it was originally broadcast in 1999 it would have been in 4:3, as virtually all programmes were back then. Now of course most people have widescreen tellies so it may have been re-jigged?
Still, it was great to see it again wasn't it?

"Everbody in the WORLD, is bent"

16:9 is correct. The BBC has been shooting most of its major dramas in this ratio for at least 10 years now, even when they weren't shown that way originally. When they showed them 4:3 they chopped the sides off. One example is the 1995 Pride and Prejudice, which looked dreadful originally, despite critical praise for its appearance - they were appreciating the costumes and set design, not the framing.

As I recall Shooting the Past was originally shown in a shallow letterboxed format, around 14:9. 16:9 is definitely the correct ratio and the programme looks fine like this.