Having looked around for some prehistoric examples, I carried on poking about and found some not-too-dissimilar medieval examples.

The earlier division into narrow log boats and broader planked lighters is maintained among the limited number of Viking age finds. Unfortunately most of these are not from the British mainland but given earlier and later examples are useful in demonstrating continuity. A larger number of log-boats and flat-bottomed boats is catalogued in the helpfully-available on academia.edu article Medieval Boat and Ship Finds of Germany, the Low Countries, and Northeast France: Archaeological Evidence for Shipbuilding Traditions, Shipbuilding Resources, Trade and Communication by Aleydis Van de Moortel. This includes some eight log boats from the eighth to eleventh centuries (she mistakenly identifies the Smallburgh boat as being fifteenth to seventeenth century), and eighteen flat bottomed barges over the same period.

The barge finds are often fragmentary, but unsurprisingly tend to be built in proportion, ranging in width from five to twelve feet, and length from eighteen to seventy five feet.

A few specific examples for which some details can easily/lazily be found are as follows, just to give a flavour:

Two dug-outs have been found in Norfolk, both about ten feet long: one from Smallburgh, dating to the seventh or eighth century, and one in Horning probably dating to the ninth century. While earlier logboat finds included a separate transom board, which could be removed, the Horning boat’s transom is part of the log.

c. 800AD – as part of the Clonmacnoise Bridge excavation, the remains of eleven log boats were found. Three of these contained woodworking tools, which may indicate their use in the construction of the bridge. Another was fitted with wooden ribs or knees, which could have acted to help the boat keep its shape, or to support additional side planking or a thwart, or as foot rests. It has been suggested that these could also have been put in to imitate planked boat construction.

c. 800AD - Rheinschiff Niedermörmter is a Carolingian lighter, about 45 feet long of planked construction. Based on photographs, she appears to be around seven feet wide, her bottom is five planks across and her sides two planks. These are joined by pairs of L shaped knees, each pair approximately four feet from the next, each knee joining all the bottom boards and one of the sides. She narrows towards either end, and has a pram bow. Although the freeboard is more limited, there is a marked similarity in other dimensions and general shape to the Ferriby and Dover boats.

I think it's time to spend a whole penny (plus £2.80 shipping) on an ex-library copy of "The Seacraft of Prehistory" by Paul Johnstone, which seems to be a slightly aging standard work (and costs rather more new!), from Amazon and see where it takes me.