I know this is an old thread but just in case this is a help, here is our experience in proving income.
We had a P60 from HMRC which was my wife's pension and would have satisfied their minimum requirements for a couple bur Guess What, they wanted it translated into Spanish officially with a Traductor-Intérprete Jurado . The P60 is so simple I can't see why it needs translated as it is fairly obvious and the S1 forms we presented didn't need to be translated.
Nothing else was of any use either, Spanish bank statement didn't have enough income entries on it, but in fact the 2 regular transfer entried were just cash paid in by me and having been told that it would be a good idea to ask them to put "Pension" in the comment when paid in I had done that.
The problem is that my four little annuities / pensions (and I do there is a significant difference tax wise) are paid into my UK bank and we just transfer money to the Spanish bank when we need it. The copies of the UK bank statement showing regular income for 4 months again were of no use to them.
So a bit of a wasted journey but we have 10 days to return and it looks like the best bet will be an official translation of a letter from our joint a/c showing my wife's regular pension income and possibly her P60 both translated although my income in total is higher. The "nice" lady told us that joint income of €8,513.26 is satisfactory for a couple and my wif'e's pension just about makes that.
Now, I know of others who have just has regular monthly funds transferred from a UK bank and called it pension and then had a certified statement from the Spanish bank and all went OK , BUT, my point is that if you are supposed to sign on the Foreigner's List when you spend more than 3 months in Spain , then why should you be expected to present them with a bank statement showing regular monthly income for more than those 3 months. And, it would be really easy to fool them, even by borrowing money and paying it in just to satisfy the requirements!
As you can guess I am a little miffed but I can understand bureaucracy and I suppose they work to a rigid framework but in our case common sense should have told them we're not here to scrounge of the Spanish welfare system.
After our first attempt on 18th January 2013 we finally picked up our little cards (plastic film on one side but the guy put the film on the back by mistake lol) yesterdsy 2 months later. I pestered them every week by email and I think I wore them down.
I would advise anyone to speak to a Gestor as we found out afterwards if you have enough in your bank for one years worth of income apparently that will satisfy them, or have regular transfers over a period of 5 or 6 months but a statement is no good. They want some sort of letter from your Spanish bank only and no-one seems to know what this letter should say (including ourt bank). Anyway we were everntually successful here in Almeria but who knows what other regions require.