The single episode here is called 'Endangered Species' and it's not on the series 11/12 or 14 PAL UK disk sets. This 'Season 13' episode was shown 5 November 2006, as a one off special - just like the previous 'Season 12's "Near Death Experience" episode from 2005. Season 14 and 15 had three and two-parter episodes respectively and so are a bit better value.
This 'A Touch Of Frost - Endangered Species [DVD]' episode is available as a cheap Region 2 [PAL UK] DVD: A Touch of Frost - Endangered Species [DVD
] via Amazon, rather than as this expensive NTSC import [but it's often not listed as Season 13 of Frost and indeed the DVD cover says 'Series 12' which can be confusing].
Apparently this 'Endangered Species' episode is on the older complete 6-12 Season DVD set, but it's definitely not on the seperate 'Season 11/12' or 'Season 14' DVD sets we have. So 5* as it's a good episode and essential for your Touch of Frost DVD collection, although the new complete Frost 'Seasons 6 to 15 DVD set' is probably the best value way to buy this episode at the moment. If you've got as far as this 13th Season of Frost, you won't need me to tell you how superb David Jason and this 'Touch of Frost' detective series are.