Heterocyclic compounds and their use as inhibitors of HCV polymersases for the treatment of HCV

A compound of formula (I): or a pharmaceutically acceptable salt thereof wherein A is a an optionally substituted 5 or 6 membered ring that may contain a nitrogen atom; the dotted line represents an optional bond; X is N, C or CH; R1 is selected from optionally substituted- alkyl, alkenyl, alkynyl group and an optionally substituted ring system; R2, R3, R4 and R5 are each independently selected from hydrogen or a substituent, or R2 and R3 and/or R3 and R4 are joined together to form an optionally substituted 5 or 6 membered ring that may contain a nitrogen atom. The compounds and their pharmaceutical compositions are inhibitors of viral polymerases, especially the hepatitis C virus (HCV) polymerase and are therefore useful in medicine for the treatment or prevention of HCV infections.