Thanks to all contributors. Thanks to the lead above to the Copper family site, I have sent an email to Jon Dudley of the Copper family. His reply came swiftly, as follows:

Dear Ian, As I normally sing this song I guess I should know the answer...so here it it is... 'Smack' clearly refers to the illicit street drug that the so-called 'Sweep' deals in, ie. his deal is as good as any other street traders. This guy is tough, he needs to keep his wits about him and can't afford at the time of dealing to have his conscious mind blurred or distracted in any way, therefore the second lyric really is a corruption through the oral street tradition of 'call me on a high' as opposed to call me on hire. I hope this answers your question in a satisfactory way ;-)

What I really think is this. Smack means hit or punch, ie. you won't get one over on this man, he's a Chimney Sweep and proud of it, so he's as good and strong as the next man, although some may think his job lowly he can give as good a smack. 'No one shall call me on hire'..well we don't have a natural service industry ethic here in Britain, and the Sweep, who's his own boss likes to pick and choose his own jobs..he won't be at anyone's call. In other words his pride says 'I don't care who you are, even if it means I lose money, if I don't fancy you or your job I wont do it'. What do you think? Thanks for your interest, Ian. Sweep is one of my favourites. Best wishes, Jon.