Yeah, that's frustrating, especially so when price trackers show it's been at £389 or £399 since last year, pretty much up until last week / week before.
Guess it's an effect of different wholesale costs being assigned to different units - you presumably bought one of the last ones of the 'expensive batch' and then it auto updated the price to the next batch which were bought in cheaper - bearing in mind the new price offering says delivery by May 14th, suggests they don't have them actually in stock yet, just expected. Is yours due for delivery sooner?
I would try as b0rn2sk8 suggests and cancel it if you're no longer happy with the price but you might find the system reprices at £428 once it registers that stock as available again.
Edit - Probably easier to just accept the price match offer now they've offered that