@daddycatt according to the Cybex website (scroll down to FAQs and Installation) the Sirona Z2 will go on the Z base although they say you might not get as good of a recline.
https://www.cybex-online.com/en/de/p/CS_PL_Sirona_Z2_i-Size_EN.html
But honestly it's a bit of a con these seats that go onto the base. If you have £320/350 to spend on the next stage car seat, I'd look at these instead which are comparable or better in features/safety and don't need a base. Then you can keep your base for the Cloud in case you have another baby:
Britax Dualfix Plus - spins, higher weight limit than Cybex, bigger shell than Cybex (so will last you longer), nicer spin system (don't need to push up child's whole weight to spin)
Besafe izi Twist/Besafe izi Turn - spin seat with built in angle control to prevent issues with sloping vehicle seats. Better leg room for child, significantly nicer to use. Currently on offer at all stockists for £309. IMO the best spin seat of all of them.
Maxi Cosi Mica / Joie i-Spin 360 / Cybex Sirona S2 - fairly standard spin seats, but quite a bit cheaper at £250-280. The Cybex S2 and Z2 are basically identical.
Joie Spin 360 / Graco Turn2Me - standard/slightly lower spec, but still decent, spin seats, half the price of the Cybex.
Axkid Move/Britax Max Way Plus - Swedish type ERF seat / best safety in the world, usable up to 25kg (about 2 years longer than Cybex). More compact to fit in car and more leg room for child. Significantly cheaper at £230-280 depending on offers. (NB Child does need to be approx the size of an average 12mo before fitting into these seats and sitting unaided).
Axkid Minikid 2 - Another ERF seat up to 25kg with same safety and comfort benefits as Max Way, plus self-adjusting headrest so you never have to guess at the right height setting. Usable from approx 9mo + sitting unaided.