Thanks for your comments/replies.
@ Anon in West Midlands, you have confirmed what I already thought. 😃
I am not really interested in the the 16" so called space saver spare option. I agree that the weight of it is a bind and the 50 mph speed restriction it imposes, not to mention the different tyre sizes 205/225 on the car. The better option is a full 17" spare wheel as in the good old days. It might weigh a bit more than the other options, but not much and I can be on my way at 70mph and have a replacement new tyre fitted, using the part worn as a spare .
The boot tyre recess is about 68 cm wide by 22 cm deep for a 225/45/R17 wheel as per the other 4 on the car.
Overall diameter of this wheel/tyre is 17" x 25.4 mm = 431.8 mm (wheel diameter) plus 225 x 0.45 = 101.3 mm x 2 = 202.6 mm (Tyre wall height top/bottom) = 634.4 mm
This gives me 4.6cm to play with on the tyre width lying down and 0-5cm over in the depth potentially fouling the shelf, which I can live with.
See https://www.tirerack.com/upgrade-garage/how-do-i-calc
ulate-tire-dimensions
Looking to find an R17 wheel for Skoda Octavia now.