You're probably right. It was made by Thames-owned Euston Films, who seem to have sold productions to other ITV networks.

ITV regions were complicated and changeable. Granada had some risky stuff, according to my Lancashire-born wife. ATV in the Midlands was pretty bland. When I moved to Hertfordshire in the 80s I was able to get, with a big aerial, London Weekend Television, which had better programmes than the 'local' ITV, which seemed to be mainly aimed at Norfolk folk.

So I could watch The South Bank Show: dum-didle-iddle-iddle-um-pom-pom...