Open the Chrome developer tools, click on network tab, ensure disable cache is checked and then hold down your mouse button over the reload icon and select “Empty Cache and Hard Reload”.
Still broken? Any errors reported in red in the console?
(And you think I’d have at least tried clearing the cache before posting here, doh! I guess what was confusing about this was that it was happening across sites, so clearing the cache was not my initial instinct. But new Chrome trick learnt as a result, so not so bad!)