How can you tell if you have one?
1 in 30 pound coins (roughly) is a fake. So I gathered together 30 of them from friends and, sure enough, a couple looked suspicious - and one was definitely a fake.
I can say that because I checked with the Queen's Assay Master!
Look at the lettering on the side in the first picture. The E and T of DECUS ET TUTAMEN* are right out of line and waywardly stamped. Plus the milled edge (those bumpy lines) is poorly defined. It's sub-standard.
The Queen's head should be vertically in line with the design on the back. The top and the bottom should line up on each side. Turning this coin around, I can see that they don't.
And here, in the second picture, you can see that the
The lesson is that if the coin looks like it's made by an amateur, it probably is. You shouldn't accept it. It is worthless and you're unlikely to get your money back.
The Royal Mint warns that the counterfeiters seem to have favoured coins dated from 2004 to 2007 recently, so you should check those particularly carefully.
Here's a link to the Mint's briefing on the issue.