Ok so I bought an S4 of eBay. Discovered it had AT&T apps etc installed. The back of the phone etc all say i9505 not SGH-i337 which is the AT&T version however O2 3G nor 4G work. EE nothing works.

All indicates to a SGH-i337 however as I say box, phone etc all say i9505, checked the IMEI it pulled up details about the phone and again says i9505 with the UK 4G Bands etc.

Could it be a case of someone flashing incorrect firmware to this phone?

Have you tried factory reset? Download Phone INFO ★Samsung★ and see what it says about country and CSC Code. You could try flashing UK firmware. Information and download links for flashing at http://androidromupdate.com/2015/07/...rmware-update/

Yes country and csc are AT&T USA. I decided BTU firmware from UK. Odin won't install it.

I think I have a US phone. It finds all uk networks but with an ee sim won't connect to ee and with an O2 sim only gives edge on 900mhz when I can get H+ and 4g here.