When I ordered my Octy vRS earlier in the year it was either space-saver spare or no spare.

I take it a full size alloy (18" Neptune) won't fit snugly under the boot liner?

Ta

You can order full size spare so I assume so

Nope, not even a 17" will fit the space. You will have to come down to a 16" alloy/rim to fit. I think that you will find that the standard spacesaver spare wheel, is not a real spacesaver type tyre, but in fact is a standard 205/55/16" tyre. Will be labelled with a max speed restriction of 50 mph, for safety.

A 205/55/16 has the same rolling radius as a 225/40/18 so I reckon it will fit

When I ordered my Octy vRS earlier in the year it was either space-saver spare or no spare.

I take it a full size alloy (18" Neptune) won't fit snugly under the boot liner?

Ta

The reason for the lack of availability of full size spare for certain models appears to be due to the fact that Skoda only supplies 2 sizes of spare - a 15" and a 16" wheel with base sizes for those rims. So a standard S trim and Standard Elegance trim can have full size i.e. match the tyres on the car whereas SE and other trims cannot. Also if you spec non standard alloys.

A 205/55/16 has the same rolling radius as a 225/40/18 so I reckon it will fit

I agree - it should fit into the well (otherwise where else would you put your punctured full size wheel if you actually needed to use the spare), but it will stick up another 20mm (205 v 225) so the boot floor might not quite sit flush. I've got a 16" Audi alloy with 205/55/16 winter tyre fitted as a spare over winter, and because the offset is ET35 against ET51 of a standard wheel the toolkit sits a bit higher and the boot floor doesn't sit quite as flush as it does with the OEM spare.