Just bought a new monoblock basin tap which has come with flexible tap tails with 3/4" end (female). Currently the cold water comes off at a tee to the tap, then goes on to feed the WC, the hot pipe goes straight to the tap fitting(previously had a 2 hole basin). What specific fitting do I need to add to the hot and cold pipes on the already existing 15mm copper pipe to be able to connect the two together?
Sounds like you have bought a bastardised tap, 3/4 is most unusual. It will probably be easiest to buy some new flexibles but take the ones supplied so you can match the thread into the tap.