hi gemini, here is the reply from surfbirds, where were they? note the comment about cambs in final paragraph! I got totally confused, as i did last time!
"They are Willow Tits.
You have it wrong about the bill - it is not pale cutting edges but a pale spot at the base of the upper mandible that denotes Marsh Tit. have a careful read of this:
http://blx1.bto.org/pdf/ringmigratio...broughton1.pdf
Willow tits also have a pale cutting edge to the lower mandible.
Shape, cap, neck, wing etc is useless in trying to identify these. Not only is it extremely unreliable and subjective, but these birds appear to be juveniles just entering moult (note the pinky tones to the legs, for juvs). Juveniles can be inseperable in these species, except for the bill feature and tail feather measurements (both of which have margins of error, around 5% and 25% respectively).
All of the pics clearly show a lack of a pale mark at the base of the upper mandible, so the chances of them being Marsh Tits are tiny. But, no matter what anyone says, you cannot be 100% sure on these images, but you can be about 95%. It's much easier with autumn/winter birds after the post-juv moult, as you have several features to base it on (mainly cheek, bill and wing).
Finally, location is also a major help these days. Willow Tits are down to around 5-7000 pairs, and their range has contracted sharply. Marsh are about 8-10 times more common, and much more widespread. But there are still lost of places where one species occurs and the other doesn't.
Incidently, I notice the OP is Cambridge-based. If these birds are in Cambs and the reservoir is e.g. Grafham then the sightings is notable indeed. Willows are extinct in Cambs."