I have a spare key, as do many people. I press the button and the car unlocks, as expected. I insert key in the steering column and then nothing - it won't turn in the lock!

The key looks identical to the one that works.

Any ideas as to why?

take a pic of the working key blade.

Send to to keys-in-the-post on ebay.
And for some money £14? then will send you a working blade.

Confirm it works and then fit to your key.

dont think they are on ebay anymore, also seem to asking for key number nowadays.... below is a link to thier web page, ask if they will do via photos.
https://www.keysinthepost.com/product/land-rover-flip-key-blade/

I might be wrong here but could it possibly be to do with the immobiliser system?
I know on my car, the keys have a wireless rechargeable battery in them which recharge when in (or near) the ignition.
Could be worth just trying a new battery in the key fob, even if unlocks the car. 2016 RRS SVR Estoril Blue
2005 4.2L Supercharged Java Black - gone but was my first love.
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If we all point our RR's in the same direction and accelerate at the same time can we turn back time?

It does unlock the car (or did last time I tried) but then won't turn as soon as you put it in the ignition.

Its something I've not looked at for a while as the other key works and its getting tired visually but I don't want to send it off to RRUK without another working key!

Another option if you're willing to pay for it is get a mobile car locksmith to come out and cut a new blade and fit it in your old fob.
I was in the same boat as you, with 1 working key and didn't want to send it off.

I did this a few years ago and it cost about £80 in central London. Pricey but it was worth it and it still works and is my main key now. 2016 RRS SVR Estoril Blue
2005 4.2L Supercharged Java Black - gone but was my first love.
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If we all point our RR's in the same direction and accelerate at the same time can we turn back time?