In 2008, the Royal Mint introduced a new design of 20 pence, as below:
Some of the old design were also issued in that year:
However, by accident, some of these coins were struck with the old obverse and new reverse and this created an undated "mule". The Royal Mint states that "this problem affected less than 250,000 coins of the 136 million 20p pieces minted in 2008-09".
When the mules were first discovered, some of the original sellers got very good prices for their coins but once the market settled down, the realistic price for one of these settled at around £50, which is probably fair.
This error attracted a lot of interest and hype, such as claims that the Royal Mint is withdrawing these coins, which is not true because the Royal Mint has declared them to be legal tender. Others have confused the activities and statements of "The London Mint", a direct marketing organisation with those of the "Royal Mint", with which it has no links whatsoever.
Even now, June 2010, sellers make the wildest of claims about these coins - though not the genuine coin dealers - and attempt to charge ludicrous amounts for this coin. Some have even tried gold plating the genuine coin to trick you into parting with more of your money.
As well as the genuine sellers, this spawned and continues to spawn countless fraudsters whose listings usually say "undated tails side" and "rare" but these sellers are liars who are trying to trick you. Some even claim that "similar 20 pence coins have sold for hundreds even thousands of pounds". You probably have one of what they are selling in your pocket.
If the coin is dated on either side, it is only worth 20 pence. If you do a search for "undated" on eBay now, in British Coins, you will see the scammers and liars.