Sorry, you're quite right. They're based on the mk.4, although the non-tilt profile of the carriages, the windows etc are more reminiscent of the mk.3. Last time I rode them you could still see the British Rail logo on the microwave oven in the café car...

And yes, the mk.4s are only used by NXEC on the electrified east coast mainline. There would have been a follow-on order of the same or similar trains for the west coast mainline, but the last recession scuppered that and then privatisation pole-axed all British Rail's plans for purchasing stock.