Is it true that people are nicking cars in car parks by copying the radio signal sent by the car?

As said you used to be able to do this with a T.V. remote that could learn other remotes. Now the Iron bar over the head or through the letterbox to grab the keys is much easier.

Easy to break into some cars by doing this, but still very difficult to drive away any modern car without a key. Usually, scanning the remote & using this to access the car is done to steal contents or steal the car by trailering it away.

Used to be in the 80's and early 90's but modern cars have rolling codes and immobilisers that physically need the key as it contains a transponder.

I think this used to happen but modern systems use a 'rolling code' ie each time you set the alarm a different code is emitted and received by the alarm remote / electronics making this type of scanning more difficult.

No. Scare stories.

Nowadays they just smack you over the head with an iron bar and steal the key.....

yes and it's quite easy