My one year old octavia 1.4 tsi was supplied with 15 inch alloys and a full size 16 inch steel spare wheel. This is labelled maximum speed 50 mph and that it should be changed as soon as possible. I asked the dealer about this speed and use restriction but he didn't know why it was there. Can anyone explain the reason for it.?

The generic spare is used on various models of car with various size wheels and tyres. Chances are it will also have a different make of tyre compared to the other three, this can lead to unusual vehicle characteristics hence the limit when using the spare.

As above.

I went for the spare wheels on both our recent TSIs but ASAP got hold of proper full sized matching alloys so I could rotate them and travel at full speed and not the 50mph/80 kph.

Yes tyre might be V rated but it is an odd diameter compared to that on the same axle as that cannot be good for braking, handling etc so hence the 50 mph if even tyre same rolling radius.

Strangely the steel spare had a Conti Sport traction AA tyre on it and the 4 tyres on the alloys were poorish Bridgestone Potenza RE050s. Fortunately the spare tyres from the 2 TSIs will fit on the 1.9D estate we have so cheap tyres really £50 for the spare for a tyre worth about £100 to me, times two.

Bought one alloy full price, ouch, another one repaired from the dealer damaged in transport on another L&K, half price of a brand new alloy and tyre.