As an addendum to this thread, which anyone reading later may want to make note of:
In the tutorial, gearbox=transmission
sump=transmission
radiator=cooler
The tutorial is here:
It fails to mention that you should drain the oil pan via the 15mm bolt under the transmission.
Then refill the oil pan.
THEN disconnect coolant line (wearing goggles! cos the rubber hose is tight to the metal pipe, and when it pulls off, it flicks ATF into your eyes :iraqi-info-minister: :iraqi-info-minister: :iraqi-info-minister: )
Why drain sump first?
It holds over 2litres of dirty ATF
The pump takes this oil from the sump/oil pan at the rate of 1l every 10s or so and pumps it around the gearbox, into the cooler then back out into the sump.
You disconnect the connection between the cooler (radiator) and sump so that dirty oil comes out at the end of the cycle.
Effectively, by not draining the sump first, you are mixing the dirty ATF with the clean ATF, and will probably use at least 3 or 4 litres more ATF before it flushes clean.
I DIDN'T do this - cos I'm a numpty :iraqi-info-minister: - and I believe it could be the reason why just changing the fluid didn't fix the problem.
Even though when I just did a sump drain and refill, the problem went away for a day.