There's no reason why an engine should overheat just because it's been left idling (especially in the cooler weather -such problems most often occur on really hot days).
Check the obvious first, which is to look for signs of a failed head gasket. To do so, remove the oil filler cap and look at the surface of the oil. It should be just as clear as if you were examining the contents of a new bottle of oil that you'd purchased from a garage. If there's 'mayonnaise' on the surface (illustrated in a fairly extreme example here:
http://i58.tinypic.com/jsgeo5.jpg ) it's a clear sign of a head gasket failure.
I had a head gasket go on my X-reg Escort about 6 years ago. I was delivering car parts to garages (as a casual job) around that time, so I had the opportunity to ask lots of firms what they'd charge to fix the problem for me. Most of them estimated between £700 and £1000 (inc VAT) but my friendly local garage did it for £487.