So to update, yesterday I drove around quite, hard at times, with AC off and no squeak. Turning on AC and driving hard, no squeak initially but after 5-10 mins of AC, you get the squeak. Again, steady 70mph on the motorway is fine, but it happens when you hit the throttle to overtake.
Today, drove around with AC on, got the squeak. Turned it off and initially there was a bit of a squeak but after 5 mins of no AC, there was no squeak when pressing on.
I do wonder whether it is AC condensation related. My problem is where do I take it to get it looked at? Car is now out of warranty by six months.