As you say Copper, along with all the pence coins they were issued in 2008 with their original reverse designs, then later issued with Matthew Dent's shield reverse. The mule being a mixture of the two.
Both the 2008 Crowned Rose and Shield reverse 20p are common, but the undated 20 pences were withdrawn by the RM and those that made it into circulation were quickly removed by collectors etc although I believe you can still find them on eBay etc. I haven't seen any minted figures for the mule.