I have an Insurance policy which covers me hiring a car in europe. It costs under £40pa and will cover you instead of topping up at the rip off rates with the hire company in Spain.
Recently I hired a car and had to pay 300 euros (taken from my credit card) as I declined their offer of extra insurance. On returning to Scotland I was reimbursed within a few days - but - as I had paid with the credit card I could not transfer the amount into my current account (Santander) and was advised to spend it!
Best to charge it to your current account, and then you will be able to access the money.
Now booking with local company in Torrevieja who are more expensive but no hassle and can refill the tank. Never had a problem with them and will now go that way instead of facing these difficult procedures at the airport.
On reflection, the car I was given had plenty scrapes and bashes, so guess they wouldn't have been able to tell if there was a new one. Didn't make up for the animosity at the airport though.