Can speaker wires be extended ( about 18in ) without loss of performance and what is the best way to go about it?
Many thanks.

ideally they should be soldered together HOWEVER

a bit of insulation tape does the same job

twist the wires together and tape them indivually

there is plus and minus for the speakers and what I do is look for lettering on the cable, then make this the plus ( connected to red on speaker ) and sometimes there is a white band on black cable which i also make positive

Thanks Niall.
I have just had a wooden floor layed and the fitters concealed the wires in the beading around the floorboards, but they left one of the rear speaker wires too short.