So glad I found this thread, thanks for great advise!
Just want to share my experience, even if some part might not apply to UK as MG's apparently comes in different flavors and specs across Europe.
I have the facelift MG5 Trophy LR (or Luxury LR as it's called here in Sweden).
First leg of a 200 miles journey going skiing with my family (yes, still winter here).
First planned stop was a Tesla SC (most of SC here are "Open for all EV's").
It did not charge. Gave a rather obscure error message, something like "Not normal power" or similar. Tried three different stalls before I gave up.
Called my dealer who then threw the theory about SC being too powerful for the MG5, which is of course not correct.
Moved on to the next DC-network (InCharge) being a 50kW DC tap.
Nope! Could not establish connection.
All happening April 1, I was getting a feeling of bad karma, but tried to think clear.
What about AC? Moved a couple miles to an AC 22kW station (also InCharge network) and pulled out my brand new Type 2 cable. No luck!
Tried another stall, locked the car, unlock, cable inserted after activating charge in app, then the other way around... Hmmm... Nothing seemed to do the trick.
Held Home button for a reboot of the Infotainment system, locked the car and waited for 10 minutes, new attempts, but the car simply did not charge.
Called MG road assist, who put me in contact with a local towing company.
Together we disconnected the negative from the 12v, but did not leave it disconnected for more than maybe 15 sec. I have now got the good advice from this thread that one should leave it for 5-10 minutes to make sure everything is cleared, but was not aware of that trick at the time.
Anyway... after reconnecting the 12v, we got several errors about different systems being disabled, so the tech girl concluded that the car had to be handed over to MG main service center in Örebro, 90 miles from Ludvika were we had the "incident".
Ended up in a Peugeot 3008 rental for the week, and had a lovely time without any range anxiety.
Mid week I had the call from the MG service center saying there is nothing wrong with the car, and that it charges from both DC and AC.
Not 100% sure what happened, or what caused, or solved, our problems. But this is my theory:
1. Tesla SC caused some sort of failure to the BMS of our MG5.
2. Apparently bad enough to prevent even AC charging.
3. Disconnecting 12v seems to have solved the BMS issue, but as it gave other warnings we did not try charging after that. That would explain why service center had no issues charging when they tried. They reported that nothing was done to the car, but apparently they cleared the errors, because from the iSmart app log I see errors that was later not reappearing.
One more thing... I also have an MG4 LR Trophy (called Luxury in Sweden), and I have charged several times at Tesla 250kW SC without any issues. It's supposed to do max 135kW, but the first DC-charge I ever did was at a SC where it maxed out at 142kW !
So, at least here in Sweden, the Tesla Superchargers are working fine with MG4, but could potentially be bad for the MG5.
/Jens