Optional substituents which may be present on such groups include for example one, two or three substituents which may be the same or different selected from halogen atoms, for example fluorine, chlorine, bromine or iodine atoms, or hydroxy or C1-6 alkoxy, e.g. methoxy, ethoxy or isopropoxy, groups.When the groups R6 and R7 or R7 and R8 are both alkyl groups these groups may be joined, together wi