I summarize below the results of my findings, and thank in particular, Chris Heard, Keith Vinicombe, Jason Crook and Peter Pyle for their very important input on this difficult challenge.
Ageing as Juvenile
This was favoured by CDRH, KEV, Jason and others and is based around the uniform and rather rounded shape to the upperpart feathers and secondary coverts, the comparatively dull speculum and the greyish tone to the yellow legs. Peter Pyle also commented on the bluish bill with tiny black 'speckling' - adults lacking the black generally and having slaty coloured bills.
Ageing as Adult
As I have stated previously, such bright leg colour is almost certainly restricted to adults although an advanced first-year drake could become this yellow on occasions. Adult drake Blue-winged Teal leg colour becomes 'duller' and perhaps more 'grey-toned' whilst moulting in July-September.
Rebecca's excellent images (and some flight shots) suggest that p9 on the right wing has been replaced, clearly indicative of an adult bird, whilst the rectrices do not appear notched as in juveniles. PP makes the point that if you enlarge the images of the tail, the central rectrices appear fresher and 'newer' than the outer feathers, again contradicting juvenile plumage but corresponding well to an adult replacing its feathers during the eclipse.
The colour pattern to the secondary coverts, with blue lesser and median coverts without the dusky wash, and the broad white-tipped greater coverts are all very much features of an adult drake, although the shape of the feathers is more akin to a juvenile.
Peter Pyle further commented ''Ageing Blue-winged Teals in early summer are always a major challenge and despite this drake being superbly captured on film, I cannot state for sure what age it is. A few more weeks and it should become obvious. Blue-winged Teals are different in respect to many other ducks in that the adult drakes do not attain 'adult-type' plumage until very late in the autumn and retain an eclipse-type plumage for much longer following the summer wing moult. The white crescent on the face generally starts to emerge during the latter half of September but often not until well into October''