Hello Josie,
This happened to me four times, and only after long journeys of say 50 miles or so. Eventually the car had to go to the Mitsubishi head office and have the drive battery connectors replaced. (at least that's what the dealer said). The first three times I had the following reasons given to me by the dealer 1. I had removed the cable from a rapid charger too early (even though, like you, I have never used a rapid charger) 2. The transit fuse might have become unseated. The dealer replaced the fuse. 3. No idea why it happened.
I got Mitsubishi customer services involved and they asked me to take the vehicle to a different dealer (now the 4th!) so that they could replicate the fault, which they couldn't. I eventually took the vehicle back. However on discussing this with the 4th dealer, they advised me that if the same thing happened again, to get the car recovered to them so that they could send the on-board computer data directly to Mitsubishi. Apparently, what all the other dealers had done is re-set the on-board computer to get the car started, and by doing so, had cleared historical data that could be analysed by a Mitsubishi expert technician. The other dealers were just keen to get my vehicle out of their premises.
The next long journey that I made, I assumed I was going to break down and prepared myself for this, so when it happened I was not surprised.
You didn't mention if you had used the Mitsubishi Assist service. I have to say, even though each time this happened to me I was extremely frustrated, they were very efficient and arranged for a hire car through Enterprise (£10.00 per day insurance fee + CDW if you take it).
So my advice to you would be.
Contact Mitsubishi customer service to a. Get a refund on the amount that you paid to the dealer and b. Log the incident.
If the same thing happens again, call Mitsubishi Assist, get the car recovered to a dealer with the strict instruction not to re-set the on-board computer and also get in touch with customer service so that they can take over from there.
Hope that the above helps.