Thanks OSS. OK ~ whats to stop me transfering from one exchange to the other. Using the above example, I should be able to transfer one BTC from Kraken to CEX, sell it right away, and make 100 euro profit.
It can`t be that easy though. I must be missing something

Nothing stops you from doing this.
It's called arbitrage and a lot of people do it especially if you have enough funds to gain enough from this 1-5% gap.
The problem is timing.
It is possible that by the time your coins hit the exchange somebody who had the same plan dumps a lot and you end up with coins selling cheaper while at the same time because on Kraken coins were under the average price somebody bought and the price is now higher here.
Arbitrage is best done if you have coins and fiat on both exchanges at the same time.
You sell in the same moment 5
BTC on one and buy 5.2
BTC on the other so overall you're in profit with no waiting time.