If you suspect poisoning of any wild creature then you can drop the specimen at the Veterinary Laboratory Agency to have a post mortem evaluation done courtesy of DEFRA / WAG. There is no charge for this. There is one in Carmarthen at:
Job's Well Rd, Johnstown, Carmarthen, Carmarthenshire, Dyfed, SA31 3EZ. Tel : +44 (0) 1267 235244. Email: carmarthen@vla.defra.gsi.gov.uk
Dr Rob Davies