I have now removed said vent! I tried pulling twisting levering etc. as much as I dared nothing seemed to move and I was quite worried that something might break. I then inserted two thin metal spatulas at 3 and 9 o’clock and this released the locking tab on one side the other side was then much easier. I have looked at photos of these vents on the miata links and they are slightly different on my Mk2.5 facelift, firstly there is no snapring, secondly what appears to be a chrome ring is in fact chrome effect plastic which is locked into the top with four small tabs, this is a solid ring so the eyeball will not come out until it is removed. The tabs were very brittle so I decided not to risk any breakages and depressed the felt pads one by one through slots on the outside. ran some glue underneath, the kind you apply to both surfaces wait 15 mins or so until tacky then press back into place. This small extra thickness of glue was enough to restore the friction on the eyeball. So job done, at last. replace vent at least 0.3 seconds! I did think afterwards if the retaining tabs had broken the vent could easilt have been retained with a few dabs of silicone. Many thanks to everyone who replied to my post.
Regards to all Geoff Peace.