I can't seem to get my flight to attack a highlighted target, they reply that they are already landing? Any ideas what I'm doing wrong? It had happened in both QMB and career.

Do it earlier if in QC. Just pressing A can do it too. They attack targets around the highlighted target btw.

I tried a quick mission again, Pembrey location, me and 3 wingmen vs, 4 109s, head to head scenario.

As soon as it started I targeted one and commanded to attack, it just gave the message 'already landing'.

(I noticed the AI actually spawned behind my location.)

I ran into the same issue in my last Campaign mission as flight leader. We reached the target area and my entire flight decided to land at the airfield there, despite only being halfway through the mission. They lwft me alone to fend off some 109s.

Maybe similar to WOFF UE, so try to set workshop setting to 'never back down' (or similar wording) or try with novice & weaker aircraft. Then launch a quick mission again.