Shudders and a bang when shifting at speeds over forty mph and drops into limp mode

Check with a dealer on this as there is TSB that is calling out a shudder from torque converter and needs to be reprogrammed. There should not be a charge for this, but check anyhow. The number is 18-027-08.

Technical service bulletin. However, it may only apply in the US. So check with dealer as it may be the same but a different number.

Sorry for butting in, but I have a comment of value to add. If that service you mentioned was for the transmission, a real good suspect is they used the wrong transmission fluid. The clue is your dandy observation that the shudder occurs near thirty to forty mph. That is exactly where the torque converter locks up to provide improved fuel economy. There are two things you can do to verify this. The first is when the shuddering is occurring, hold the accelerator pedal perfectly steady, then with your other foot, lightly tap the brake pedal for an instant. That will cause the torque converter to unlock for two or three seconds, and engine speed will increase about 200 rpm. The other thing is to observe the shudder does not occur at forty mph when the engine is still cold. If you start a cold engine that has been sitting overnight, then get out on the road right away, the torque converter will not lock up for the first five to ten minutes, so no shudder will occur.

"TSB" is "technical service bulletin". Those are quite different than recalls. Service bulletins are issued to identify the causes and fixes for problems that have been identified as common, but have elusive solutions. They are intended to reduce the need for each mechanic to spend a lot of time searching for the same thing.

The service bulletin HMAC300 found involves the air conditioning compressor cycling on. If this is related, the shudder may not occur when you have the AC turned off, and the heater is not in "defrost" mode. The AC system runs when defrost mode is selected.

Part of the problem is you have a 2.8L which I am suspecting is a diesel so it may be different from US spec. However, the US spec is automatic transmission fluid Plus 4 or equivalent. If you have a manual for this then check in there as it will say in there as well.

We don't know prices in the UK doesn't sound bad though as I know prices there are much higher than in US