This is the cockney rhyming slang version of having a gander, to look at something. Though it may seem strange at first, it’s pretty simple, it is constructed out of the expression’s second word, in this case the way ‘hook’ rhymes directly with ‘look’ however, perhaps contrary to expectations, the word ‘hook’ is often removed, so you may hear someone say ‘have a butchers at this.’

But like most things cockney, it’s becoming less popular.