After draining the D3 and replacing with D2/Transmax M, things still weren't as they should be; rough changes, loss of forward drive occasionally as well as still hanging on in top gear. POwer wasn't up to what I expected based on others I've driven so today we removed the base plate off the transmission.
13 Torx screws had been judiciously tightened to about double recommended torque by some gorilla which necessitated the removal by using a small chisel to firstly slacken them as the heads had also been chewed in over half and these were replaced by using hex headed stainless screws.
Once removed, it revealed a fairly clean interior with what appeared to be a fur ball on the base; this was a magnet that simply sits on the inside of the base plate and collects any metal particles.
As you look inside, there are a couple of alloy castings looking at you. The lower one is the one we removed. It's held with 9 screws which have varying lengths. It is imperative that they all go back where they came from, so a piece of cardboard with a clear outline of the casting and direction referred to is marked and these 9 screws systematically removed and placed in correct order. When removing the casting, try not to break the gasket or it's a fiddly job cutting a new one. Inside you will find the filter; remove wash, replace and reassemble. It looks very much like an LHM in tank filter but if partially clogged, can cause feed problems to the pump. Refitting is the reversal of removal (as they say in the classics) which includes the moving out of the way of the pressure regulator to be able to remove the baseplate.
In reading the factory manuals and Haynes, great emphasis is placed on the "kick down" adjustment and according to a guy in the UK, an auto specialist blamed, lack of fluid changes, use of Dex3 and kick down cable incorrectly adjusted as being the 3 main causes of box failure. Others also have made an issue of this kick down adjustment.
The adjustment is based on the setting on a bowden cable on a quadrant on the intake which is set opposite the accelerator cable so that when the accelerator is flattened, it will pull the kick down cable which in turn operates a mechanism inside the transmission and causes the transmission to downshift. Amongst the settings is a ".5mm clearance" between the nipple set approx 39mm up from the end of the cable and the adjustment end on the outer cable. This has to be checked when disconnected from the quadrant, but in my case, the nipple sat hard against the end of the adjuster so was in fact holding the kickdown slightly open at all times. The reason we can't fathom but it has to be either slight wear inside the selector area, incorrect adjustment at the gearbox end or faulty positioning of the nipple. Regardless, shifting the position of the nipple seems to most logical as it's a half a day R & R the kickdown cable into the transmission. The reason for clamping a nipple there seems a bit overkill as its main purpose in life is that of a marker, yet if it's either in the wrong position or if wear etc causes it to hit the stop, it then begins to create what can become terminal issues with the transmission. As my friend says, for what it appears to do, a black dot from a marker pen would do the same thing. If I find to the contrary, I'll advise.
Moved the nipple so it went as per specs (.5mm) clearance, then adjusted the accelerator cable, set idle speed and then refitted kickdown cable.
Took for a test drive and the car is even further transformed. Well worth checking this cable for correct adjustment and if the performance is a bit sluggish, also do the filter and set all the adjustments; an afternoon well spent.

Alan S