Depends a bit on whether you are buying privately or from a dealer.
Dealer wise, I would say now is a good time as coming up to quarter end and possibly year end depending on their FY so they will have targets to hit. Also a fair chance that there is less demand due to most people getting ready for Xmas etc and less concerned about buying a car. Would be really interesting to see stats for dealership footfall by week of the year or similar.
Privately, not quite as good for the above reason i.e. people are busy and typically would be needing to buy a car to replace any car they put up for sale (i.e. less stuff being put up for sale because they don't want to be hunting for a car themselves). I'd say waiting for January when people are skint might be better, may be more opportunity to drive a hard bargain from people with bills to pay.
Another factor to consider is the trade price of cars (based on CAP valuation) typically changes every month so arguably if you buy a car at the start of the month it will depreciate marginally slower than if you buy it towards the end of the month, but probably not worth worrying about and could be factored into prices anyway.