Simon Mark Brambles, probably from Malaga.
I'm casting around a few forums etc to try and contact him. He's not done anything wrong.
He has a Unicaja bank account, opened in Malaga. I have a Unicaja bank account as well, but not from there.
I have a Spanish phone number which I believe he had before me. Strange I know but I keep getting messages from Unicaja with a security code and I think whenever he wants to check his bank account online, he can't get in to it - because I get the security number.

The problem is, and yes, I'm in contact with the bank to try and resolve this and they are supposed to be trying to contact him as well, but each time he tries and gets a code on my phone, my access to my bank account online seizes up because we have the same phone number.
He more than likely has a new phone but hasn't updated his bank details - I'm thinking it's just a fairly dormant account from years ago and mainly banks elsewhere.
I'm trying on the expat forums on facebook etc as well.

I would switch banks

Don't want to. I have a relationship with those at my local branch and I don't want to start from the beginning again with another one.

I'd get myself a cheap payg phone and just use it for that account

Yes, that could be one solution.