What makes the seeing sub-par this winter? :)

[blusky]

E.g. dust/smoke/exhaust particles or jet stream? There seems to be a correlation with the amount of cloud: the more the cloud the better the seeing?

http://www.7timer.com/index.php?product=astro&lon=-0.12768310000001293&lat=51.5073346〈=en&tzshift=0

[LukeSkywatcher]

the more the cloud the better the seeing?

Absolutely, if observing clouds is your hobby.

[Moonshane]

it's a well known phenomenon that high thin cloud indicates very stable air and the seeing is often better in such circumstances allowing high power observing of planets / doubles etc. very clear air is often less stable but highly transparent and makes viewing of faint targets at lower power more appropriate. it's only rare occasions when seeing and transparency are both good.

the jet stream and other factors you mention certainly affect seeing quality e.g. http://www.cloudynights.com/ubbthreads/showflat.php/Number/3453815