Here is a tip, always start the first letter politely, if you end up suing them, this letter will be shown at Court and you don't want to come across aggressive from the get go. You want to be seen as the victim and be hard done by so the Court will compensate you, so you want to start out polite but state what you want at the same time.
I'd send them something like this, send it by post, stuff email. Mark it urgent, or it could end up in the pile of post in the mail room for weeks.
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URGENT
Dear Editor
Re: Photo in Article
I refer to your article on "So that's what little boys are made of" and write to inform you that I am the photographer and copyright owner of the photo titled "Splashing out: He's in for a real shock". I enclose an invoice in the sum of £300 and look forward to receiving a cheque made payable to Mr John Doe in the next 14 days.
Yours faithfully
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