I am swapping out two metal light switches for plastic ones and notice that there are three wires, red, blue and yellow plus the metal earth wire all attached to the inside of the switch I am removing. Is this hence a two way switch and if so can I buy a plastic one that can make use of the earth wire or do I need to put that into a termination block?
Thanks
Its not clear from your statment but if you only have one red, blue and yellow wire into a two gang switch then that suggests that you may be missing a a cable or two. One set of three core would normally suggest that it forms the end part 'strapper' of a single gang two way switch. If it is not part of a two way arrangement then you will need to investigate further.
Oh and I hope you do not intend to utilise the earth for the missing cable. Remember you cannot
'make use of the earth wire' other than put it in the assigned position in the back box or if not available put it in a terminal block.
But before you start messing about with your lighting circuit you need to sort out your
'faulty' FCU first don't you?