Ryder
Having a senior moment
Ref Carpy clutch issue.
Your description of the problem with your clutch is common problem.
Without seeing the item in person I can only be 99% sure it is the problem.
You have incorrectly set the clutch hight and in doing so incorrectly set the clutch pedal without having enough free play.
You need to readjust the pedal hight and make sure you have some FREE PLAY in the pedal. If you do not allow the master cylinder to completely bottom out then pressure is retained each and every time you press the clutch. Long journeys and vibration will keep up the pressure.
You will find the clutch will slip more and more until you fix the free play issue.
Unless you have done something really crazy then this IS your problem. It is a classic mistake made by home mechanics and one I have seen and rectified personally many times in the past.
The gear you are in has no relationship to the problem other than the fact lower gears will have an easier time accelerating the car and at present you can transmit enough torque for this to happen. Unless you fix the problem you will find the clutch will slip in lower and lower gears until you burn out the clutch and then that cheap master cylinder has cost many times more bucks than you originally planned.
You will find the procedure for setting enough free play in all good haynes or land rover manuals.
Good luck.
Blimey two posts in 16 months but every one a gem of wisdom!
Always knew you were there just watching mate! There's a name for people who like to watch ya knows!