We at SERC [Somerset Environmental Records Centre] are certainly interested in all things Somerset. We have arrangements with organisation such as the Vincent Wildlife Trust to pass data to them. This ensures that data does not get duplicated. A number of years ago we worked with VWT on Stoats and Weasels in the county.
I can tell you that a population of non ferret cross Polecats is know to exist further to the east but as had been said it is an under recorded species that has been spreading across the county in the last 10 years moving south and west.
Just as a note there are about 100 terrestrial mammals in the UK. SERC has records of 14,011 species in Somerset. Please send us your non bird records.