I need to clear new plot ... cardboard ?

I'd have a recce with my spade! I dug a bit of mine yesterday, and, although wet it wasn't sticking together in claggy spadefuls. You'll have to wait if yours does, though.
You can still cover bits with cardboard and make holes through it to plant bigger stuff like squash/courgettes later.

Have you considered using the No-dig system. You lay cardboard down and cover with well rotted manure or compost and then plant into that. Each year you simply recover with another inch or so of compost, so you never actually disturb the clay. Have a look at Charles Dowding's website - he is on clay. He's also got a number of videos on You Tube which makes it pretty clear.

enough to completely cover your plot 4 - 6ins deep -

That's only about 50 cu metres in my case!

That's a thought snowdrop, a big shed for making tea, a couple of large compost heaps, several water butts, a big greenhouse, a patio/seating area where I can drink the tea, then I'll probably need a portaloo, cracked it - just room for a small raised bed for the wife to manage !