Thanks for the reply, Dave. Your comments mirror the problems that we had with the old engine.
What I am struggling with though is whether or not to get rid of the car bearing in mind that we've only had it since last July (and hadn't planned on changing it so soon!), it has only covered 33K miles, we're only the second owners and we like the way it drives (it is an M-Sport). If the new factory engine is exactly the same as the original then I am not confident. However our warranty company who paid for the replacement are of the opinion that the new engine will have certain mods (the old engine "was made of cheese, we doubt that the new one is"). I'm also advised that the problems with the N43 arise partly from the condition-based servicing regime which can see the car running on badly contaminated oil which contributes significantly to the failures that have occurred.
As I don't cover more than 5K miles per annum and have always had my own cars serviced at least annually, inclg oil & filters, I wonder if the risk is going to be reduced. I'm also advised that there is a 24 month warranty on the new engine from BMW which doesn't guarantee anything, I know, but it could cover against a similar failure in that period.