Ok, the engine ECU was replaced with a brand new one from BMW at a cost of £1,050.00. Now that it has been done, the garage said the 'voltage' is what it should be - before it was really low.
I have since done 155 miles in the car - so far, so good. But, I'm not getting too excited just yet as the light has come on in the past at 80, 157, 278 and 378 miles. So, if I can get to 500 miles I will start to relax and think it won't be back.