The Speckled Wood situation at Swanton makes me wonder if you have different population levels of the two populations. It sounds (since you are seeing clear broods in the transect) like you mostly have the "over-winter as caterpillars" type. If you had both types in equal numbers you wouldn't be able to see peaks in the transect so clearly. Perhaps something hit the "over-winter as larvae" population in the mid 2000's, or perhaps you were mostly colonised by a single type? Intriguing.
In other news, Swallowtail have started to emerge - two reports so far this week.
So reported so far this year we have:
Dingy Skipper, Grizzled Skipper, Large, Small and Green-veined Whites, Orangetip, Brimstone, Swallowtail, Small Copper,
Common Blue, Holly Blue, Brown Argus, Green Hairstreak, Small and Yellow-legged Torts, Red Admiral, Comma, Peacock, Painted Lady, Speckled Wood, Wall and Small Heath
So despite a slow start, we seem to be back on track. The first BC field trip of the year is today at Stoke Ferry: full list at
http://butterfly-conservation.org/291/norfolk-branch.html