How to unlock iphone in another country?

Due to my last iphone breaking and costing more than it was worth to get it fixed, I decided to treat myself to an iphone 5s which I bought on eBay from a UK seller but it is locked to vodafone. I am a vodafone customer in the uk and also in Spain, where I am living for a year, but my UK sim is a micro sim and doesn't fit in the SIM deck. I am not due back home till December so can't do anything about it.

The phone was wiped before being sent and when I put my Spanish vodafone sim in, which is pay as you go, I cannot set the phone up as it says it's invalid. I don't know the account details of the last owner and don't really know what to do in order to unlock my phone to allow me to use my Spanish SIM. Please help!

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