Geekyflyer - the better afternoon weather is almost certainly due to mist/low cloudbase you see in the cooler months.
Worth noting this will be an issue for training from Denham, since they have to cross the Chiltern ridge to get to their training area to the NW. It's only a small topographic feature, but at this time of year is a real mist trap, especially to the NW side (in my experience....).
It can be very flyable to the SW of London, but very nasty indeed round the corner to the NW. This has happened several times in the past few weeks.
There is a theory this is not helped by the Didcot Power Station belching out huge amounts of water vapour.....it is interesting how Benson's and even Brize's TAF/METAR appear more pessimistic than Heathrow's and Farnborough's.
Is that due to the Chiltern ridge effect, Didcot, or the fact they have human forecasters on site who are able to take account of local knowledge??
Discuss!