The Cheese and Grain might be a pleasant venue in September, but not in the middle of summer, at least not for me.
The rest of it depends on what's on your shopping list, your luck, and how sharp eyed you are. Anyway, there's another side to it, like bumping into people you know, and chatting with the stallholders about some of the stuff they are selling. Obviously at the smaller rallies there's just less stuff.
There used to be one in Almondsbury, just north of Bristol. It was a bit bigger than Chippenham, but not much. I had a few good buys there but it seemed to throw up a lot of curious things. One year there was a bloke selling off a collection of those little Japanese multimeters sold in the electronics rags in the 60s and 70s.
There's another small rally in Weston-Super-Mare. I rate the small rallies as worth no more than an 80 mile round trip.
Has anyone any thoughts on Flight Refuelling? It's not that far but it's a tortuous route from where I live.