I sometimes take my grandsons out in my M3W when it is having a holiday in England. Extraordinarily, I found a very helpful bunch of assistants at the Halfords in Brighton, Sussex (I am sure that the M3W helped). They tried out all sorts of booster seats and child safety seats, pretty much every one they had in stock. I had already tried the Recaro dual fit seats we use in other cars, which can be either belt or ISOfix located. It was far too bulky for the M3W and did not begin to fit. The only device we found that seemed to fit properly and provided security for 5-8 year olds was the Halfords Basic Booster cushion, with belt location hooks. Now I know this does not provide the sideways head location that you get with the Recaro but it is a lot better than nothing and at least fitted in properly, which the larger seats just did not.
Hope the repairs go better than mine did at the south east of France agent, who took 11 months to wreck the car. It then had to be trucked back to Malvern to be re-manufactured, which to give Morgan due credit, they did free of charge.
Wilson