Can I please ask, what is the difference between a single and a double pole 13A switched socket?
With a single-pole switched socket the switch opens only the live conductor. With a double-pole switched socket it opens both current-carrying conductors. There's no particular advantage to the latter arrangement.
Thanks again TW.
The reason I considered changing all the cables whilst extending the circuit is because I believe it is about 30 years ago when it was last rewired.
So long as it's not been subjected to direct physical damage or deteriorated due to excessive heat or something similar, there's absolutely no reason why PVC-insulated cable of circa 1980 should need replacing just because it's 30 years old.