There is an account in BSBI News No. 125 January 2014 pages 37-38 by Ian Bennallick and David Pearman of the discovery of the true service tree (Sorbus domestica) growing wild in Cornwall on a cliff in the upper Camel Estuary.
The species has been known in wild situations in the British Isles since 1993 when it was discovered on cliffs in Glamorgan, south Wales. After this it was also found at sites in the lower Wye and Severn Valleys and the debate continues as to whether it is genuinely wild in the UK or has arisen from ancient introductions.
The Cornish example perhaps strengthens the case for its being a native and some of the older literature does mention Cornish sites for the species, though none are specific enough to be followed up.