Specifically – 135×12. The online retailers state 142/135×12 – does this mean its min 135 with spacers(?) to go upto 142? Also does it work with a maxle (12 of course)?

Yes to the spacers question. Yes to any 12mm axle (there’s 3 or 4 different systems iirc)
Easton also released a free fix for the hub problems with the rear wheels a few months back.

Mate, last thing you need to buy are non-adjustable wheels (axles/spacing etc)…

What do you mean? They don’t have adaptors etc for different wheelsizes a la hope?

I had a set of Havens and was really impressed. Light, good width gives a nice tyre profile, tubeless set up was really easy, hubs look amazing, I bashed em around a bit and they stayed true, despite my 6,2″ 90kg bulk. I never had any hub problems (had to adjust the preload once) but then I only had them for about a year before my bike was nicked. I’d definitely get another set.

What do you mean? They don’t have adaptors etc for different wheelsizes a la hope?

Not when I looked (carbon Havens), you had to buy the ‘right’ wheel.

The rear wheels come with the spacers to fit 12×135 or 12×142.

I got the rear hub upgrade kit off Extra, but having no issues with the original stock internals, so I havent swapped them out yet.

Front 20mm hub can be converted to 15mm too, Extra emailled me the kit no.

Love mine, nice and light, look bling, no issues and still totally true so far. Glad I took a punt and ignored the horror stories.