This was an interesting find by Keith Crowden. Neither David Mitchel nor Pat O'Reilly, proper mycologists, who led walks on 'Fungus Day' remember having seen this previously.
As Hornbeam, Carpinus is native to south-east England, the majority of records are from here although the first British record in 1884 was from Cumbria.
The formation of peg-like structures lifts the bark. However the fungus starts on the bark surface as a blueish crust and, for a long time was considered a different fungus, called Peniophora pseudonuda. The image below shows the fungus on bark but this has grown bumps showing how variable the species is. Hornbeam is present in Spring Woods and Trawscoed Wood (and probably elsewhere in the Botanic Garden) so, once you got your 'eye-in' the fungus could be found easily on fallen branches.
Sorry this image is of a different species!
This is Phlebia radiata Wrinkeled Crust. This may be found throught the year but mainly over the winter months, Autumn to Spring.
These two species were close together on the ground, and as there was some similarity in colour, I jumped to the wrong conclusion.
I added the comment below.