we have an economy 7 system, so a hot water cylinder with a cold water storage tank above it.
i replaced the inners of the basin mixer tap, and whilst the cold water is fine there's no hot water here.
there's also no hot water from the kitchen mixer tap, or from the bath mixer... BUT, there IS hot water coming out of the shower (which all uses the same feed as the bath).
I've read somewhere that this could possibly be an airlock, am i on the right track?
Yes you are on the right track.
If you have a mixer tap in the kitchen try sealing the end of the spout with your hand, turning on the hot water then the cold this should blow out the air-lock.
Some taps have a dual pipe feeding the spout - if you have this sort of tap find a plastic bottle which is a good seal on the spout and proceed as above.
If you have separate taps connect the together with a bit of hose and again proceed as above.