Thanks to all and sundry.
I doubt it's moisture, because the camera lives in a box of silicon (silica?) gel when it's not out and about.
Neither does it live in an air conditioned house. Really, there's only one temperature here, and the camera should be used to that.
The lens is a Canon 18 - 135, about two years old.
I do regularly clean lens and any filters, at both ends!
Anyway, you have given me research to do, and I shall follow up your suggestions.
No 'service tech' within miles of here!

Have to leave you now and go home, where I have no internet. Thanks, will follow up those links tomorrow.

I had not had the presence of mind to try two pictures in quick succession: that's an idea.
LENS HOOD? Why didn't I think of that. It's always sunny here, and I don't use one. BUT: I don't usually get this problem, sun or no sun; never with the other camera (a 400D with any lens), and I can usually see flare coming in the viewfinder and do something (repositioning, hand...) to cut it out. Flare normally looks different, but this is an interesting lead.
Thanks Steve (?) for some relevant links. Easter Monday reading...
I'm really most grateful to all of you.

...I took many more pictures on the afternoon of the picture in the original post, before and after. All were fine.

I confess I have not tried deliberate substitution; but the problem is so intermittent / inconsistent with this one lens that I haven't thought it worthwhile to try others. NO visible dust / smudges anywhere, on lens, filter or, as far as I can tell, sensor. Some pictures have a slight white (usually) cast from a light area to a darker one; usually, but not reliably, in the middle. Most don't. I'm baffled.
So is everyone else, by the sound of it. THANKS for trying....

I think I can rule out a dirty lens: I keep the same one on the camera for 90% of the time and am careful to keep it clean and dust-free.

Can anyone please tell me why there are 'foggy' areas in this picture - i.e. the 'overspill' from the white shirt of the man on the left onto his hair, the 'fog' around the arm of the man in the blue shirt, and the reddish 'wash' from the woman's shirt onto her face?
Taken on a 60D, which has this as an 'intermittent' fault, of which this is the most extreme example. (If it's not my fault.)
It also appears as a v. occasional vertical light band down the middle of a picture (including uncontrasty landscapes) - but it's rare.
Grateful thanks in advance: I may not be able to return to this computer for a while.

(Download)

Dear All
I've been spying on you for about two years, and am so very grateful to so many of you for the tips, links, advice, benefit of experience, entertainment....
I wonder if you could help me with this,
I may access UH via my personal email on one of three office computers (no internet at home).
1. When I registered, the page entitled 'Photography Forum Digest' (With Contests / Main / General...) sprouted little green tags ('New Topic') at every item. I'm not sure what the point / meaning of this is, nor do I know why I only get it on one of the three computers. Aren't ALL topics new each day?
2. If I look back at a previous day's lists on the 'Photography Forum Digest' page, they are not quite the same lists on all three computers any more - the computer with the green 'tags' omitted at least one item (under 'General') that one of the other computers for the same day had. How come? I don't want to miss anything. This was something that I specifically wanted to return to the next day, but it was only to be found on the non-'green tag' computer.
3. How do you tell when a topic is 'finished', i.e. it is not worth commenting any further; all replies are 'in'? At the expiry of the day it was posted? Or do topics extend for several days? I do not see dates.
4. The 'green tag' computer (only) now offers me 'Upcoming Topics'. Are these for the next day? They are not sorted into categories. Are they sorted by the administrator? Does the writer get to direct where his post goes?
5. What does 'OP' stand for?
Many thanks in advance (I cannot usually return to UH on any given day.) My apologies for being so obtuse!