Which would be a better time to buy a used car - August or September (or October?)

I'm thinking that the dealers (and private buyers) might be feeling the pressure of lack of interest due to the summer holidays, but on the other hand there will be lots of turnover from the plate change so there might be greater volume in September pushing the prices down. Or would this effect filter into the 2nd hand market in October?

There's a few posts about best time to buy a car, but there wasn't much advice about August vs. September.

Thanks,