Zero trace of this registration on the DVLA, which leads me to believe this is actually a Hong Kong plate – the 'ER' prefix was issued there in 1990, which is correct for this generation of 900.
When I saw the car, I had a big doubt about the origin of the number plate, it didn't quite seem to be UK.
So I took some more photos of the plate details. There's also a French insurance sticker (out of date) on the windscreen, which I didn't understand. On my photos of the details of the plate, I checked the name of the manufacturer, and indeed it is a Hong Kong company according to my research, but in its catalogue it offers British plates. There's also the code "BS AU 145e", which corresponds to British standards. So I thought it must be a British plate (seeing a Hong Kong plate here seems so improbable!) and uploaded it that way.
Thank you for your vigilance, if anyone can provide additional information or make the plate change?