The "True Service Tree" or "Whitty Pear" is Sorbus domestica L.. You can find a photo of it here or in the Visit Details. A detailed description of its history, uses and range can be found in Jennings* and Claxton*. The tree is not actually an Ash (Fraxinus), but a close relative of the Rowan (Sorbus aucuparia, the so-called "Mountain Ash"), and is a member of the Rose (Rosaceae) family. Other close relations, which bear similar fruit but look nothing like Ash, include the Wild Service tree (Sorbus torminalis) and the Apples/Crabapples (Malus).
The tree is probably one of the rarest that might, potentially, be indigenous to Britain. There are, apparently, only 23 mature wild examples currently known in Britain* and it is one of the most endangered species of tree in the country with a plethora of plans being developed to preserve it**.
It seems likely the tree was rare even when our list was compiled - why compare it with the common Ash if everyone had seen it? Patrick Roper* has suggested the tree is a Roman introduction, and this doesn't seem an unreasonable conjecture as they used it elsewhere to make drink (see Myth). Whatever the truth of that possibility, it certainly appears that the tree is reluctant to propagate by seed under the current British climate. For example, the concentration of trees in the Wyre Forest area are mostly cuttings from one lone specimen, first mentioned in 1677*/8*1. That said, it seems the tree can last at least 400 years, sometimes propagating by root* and the British climate has changed a fair amount since the list was put together.