In reply to Luke90:
EHIC reduces the cost of medical care to what locals would pay, you'll need to check if it's free in Spain.
If the top up insurance which you are talking about is part of the policy from the same company it might be ok.
A friend of a friend had an accident in Spain a couple of years ago the various things which could have been costly:
Rescue
Medical Treatment
Hotel Room for his Girlfriend to stay out there with him over Christmas along with his mum and dad.
Repatriation (private aircraft with a doctor, some serious cash probably got burned here)
I think he was insured by Club Easy, apparently they were really finicky with paying out for everything but apparently they did.