We did the remove cowls, check thrust etc and refit on Johns advice.
No difference with or without.
Talk to John, Steve or Ian.

Purely as a diagnostic tool,---- can I again suggest you quickly remove the rear  "lift cover" over your lift intake, and see what happens?
This is NOT any form of criticism of the  previous owner /  or present cover, Nor of you or your operations. It will immediately allow lift air a more straight forward route into the fan area. There is a simple rule that ALWAYS fits PHAN PHYSICS---->-
Let the fan see the wind.
Works for thrust and lift.
If you revisit your fan intakes,-----

Thrust intake area 99% clear of obstructions,
Lift intake  area "considerably obstructed" indeed the fan is almost invisible.
It's so simple to remove check and replace. Go out - recheck.
I agree with your thoughts that having the roof up may be worsening this problem by smoothing out the air flow when up and directing the ambient air ACROSS the almost occluded lift air intake.
It currently LOOKS very nice.
IF removing the lift cover for a few minutes - cures your plough in problems then you could leave off for the time being, or rethink the aerodynamics of the cover to HELP lift air see the fan, rather than its current situation. If you are DEAD against this- please let me know so I can forget it!!!!
I reitererate- THIS very thing has happened to me on  a front lift Falcon  that I modified to quieten and improve looks.
It was quietened BUT revs needed to be higher to attain enough lift and noise returned, but lift was strangulated.
Removed cover- it was better- lesson learnt. I pass on this experience FOC!!!!
Hope you and Beccy are enjoying the Spring returning to one of the finest areas to live----ploughing or Not!
KRs Nick Of Yorkshire (Central England)

Al
Sometimes you surprise me.
If the craft went into wind, in chop and soaked it up with no problem.
The craft is doing its job properly.

Then you turn round and go downwind.
The craft doesn't know the difference it will go and soak up the chop just the same as it did going into wind.
So
If you are going downwind you start feeling like you are going too fast.
What do you do?.......Reduce power.
Then what happens.

Back to the same thing happening again with the Surveyor.  Last time out it flew faultlessly.  Today was a different story!  Yes today was a little breezy and the water was a little choppy, but flying into the wind the Surveyor took it in its stride.  The return trip though was at times very eventful.  Yes it did plough in pretty violently, Becky was off her seat and the full 20 ltr fuel tank flew forward about 2 feet, only thing stopping me from flying off my seat and ending up on the floor was me holding on to the steering wheel.  It done this several times on my way home.  The lowest the RPM went was 2800 rpm.  I also tried keeping it over 3000 rpm and trying to slow it slightly by opening the brake flap a fraction, that didn't work either.

One thing all the lift issues have had in common was the roof has been up at the time, so I'm now wondering if the roof is causing lift issues when travelling down wind, either by not allowing enough air to the lift fan through the lift fan covers or if its something to with the way the wind hits the roof from the rear.  I have no idea, so open to suggestions