Might it be Circus from 1975? This is the only reference I can find to it:
BFI | Film & TV Database | CIRCUS EPISODE 1 (1975)
E.
Can anyone identify a children's TV thriller (probably BBC1) from the early to mid 1970s, set in a circus. I think it was Gerry Cottle's circus. I remember a scene where the coloured brake hoses of an articulated circus trailer were cut as part of a campaign of sabotage which also involved a fire in the big top.
Might it be Circus from 1975? This is the only reference I can find to it:
BFI | Film & TV Database | CIRCUS EPISODE 1 (1975)
E.
That looks plausible. It's a shame that the BBC's Production Database isn't online any more. They closed it down to public access a few years ago after a trial period. While it lasted, it was a great source of information about cast/crew of BBC programmes from days gone by.