I've never seen one myself but the Vincent Wildlife Trust are interested in sightings and carcasses.
As Pete says ,the Vincent wildlife trust are the guys who survey these and the distribution map on their website shows them having spread down into central Southern England but not Somerset - but that map is dated 2006 - 9 years out of date ! 5 years ago picked up one at Wrantage which was assumed(?) to be a hybrid, have found 1 nr Ilminster , and no less than 4 now on the road between Hambridge and Curry Rivel in the last two years.One was a night time sighting of a live one ,and the other 3 were all found early in the morning or after dark indicating that they hunt by night? Just this week picked up a beaut - undamaged freshly dead and measured 25" nose to tip of tail as did one of the others. Photos of 3 of them on request if you email me - all legs/feet black, white muzzle,ear tips and small white tufts at start of eyebrow, no white patch on throat/chest.Black 'long' fur with yellowish underfur...also generally covered in pale hair lice and ticks after rigor mortis has set in looking for new hosts ! All these were found along rabbit infested banks/field edge areas.
I don't know who Vincent wildlife trust are, but Somerset Wildlife Trust are the consultative body to who polecat sightings should be reported.
As far as I am aware, strictly speaking the (possibly statutory)keeper of wildlife records in the county is the Somerset Environmental Records Centre, an arm of the Wildlife Trust, to be found at http://www.somerc.com/homepage/
The only reference to polecat on the SWT website concerns their trainees seeing a captive one. The Somerc site also makes no specific mention of polecats. By contrast the Vincent Wildlife Trust seem to be a mine of information on the polecat and other mammalian species. I have no axe to grind - they just seem to be 'on the case'.
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.
Data Manager, SERC
Interesting article in British Wildlife Vol 20, no 4 april 2009 237 -244. The Polecat in Britian -continuing recovery.
Amongst other things information there about differences between true Polecat and interbreeding, (Polecat-ferret).
Havn't checked VWT website but they feature heavily in the article, presumably some information on their website about pure polecat and interbreeding.