Is there a rare undated 20p in your pocket?

How it happened: In 2008 there was an incredibly unusual lapse in the strict quality control at the Royal Mint. A batch of 20p coins were issued with mis-matched sides. The result is a number of 20p coins have entered circulation without a date.
Read more about how it happened here >>

Identifying one of these rare 20p coins: Not all the new 20p coins are error coins - in fact, very few of them are. To be an Undated 20p it must have the new design on the front, and on the Queen's head side the date must be missing.
You can find help identifying the right coins here >>

The London Mint Office is no longer accepting applications to register your undated 20p: Unfortunately, the initial allotment of places available for you to register your 20p piece have all been allocated. However, the quest for the undated 20p continues so remember to keep a look out for one in your change!

What is an undated 20p?

This is an error variety of the new design 20p piece - note that the Queen's head side does NOT have a date whereas the regular version carries the date '2008'. Both sides need to match the photo, exactly.