There are several "open source" maybe asking for a donation applications that will allow you to read what the sensors are seeing:
https://rovermems.com/
https://andrewrevill.co.uk/MEMS3Mapper.htm

They would at least allow you to see what is going on, but you do need to buy a cable.
At this age of the car, I would suspect wiring issues may start showing themselves.

There are a couple of threads in The Mini Forum UK, but there is a lot to dig through:
https://www.theminiforum.co.uk/forums/topic/20533-having-problems-with-spi-single-point-injection/

My SPI has been acting up recently. A couple of weeks ago, I had a no start. Got it going by fussing with disconnecting and plugging connectors. Not sure what worked or not. It feels like an electrical issue. When I press on the gas pedal it stumbles. Clutch in, I try to rev it. It takes  repeated feathering of the gas pedal to get it to smooth out. I hot wired the fuel pump, so that is good.

Any idea what the culprit would be?