I have used an unattended petrol pump and I don't understand the transactions in my pending list.

Since the retailer doesn’t know how much you plan to pump, some will authorise £1 to check that your card is valid. Others will reserve up to £100 then reverse it, and replace it with the final amount pumped when you have finished filling up.

You may see these transactions in your pending list if you have used an unattended petrol pump.