Is it true that the springs fitted to the sport cars were 15mm lower? Looking around it seems that Eibach 30mm lower are the main choice - but if there are such a thing as OEM 15mm lower are they readily available? And are they very expensive?

No, just that we can't find the part numbers! You could try ringing a dealer and asking them, but give them my VIN number for them to look up.So from that can we deduce there is no longer any stock available from bmw?
Okay.. using @GazHyde gazza's sport models VIN l have just got reply from Sopers:
Hi Alan,
The prices for springs are:
Front springs - £83.48 each
Rear springs - £203.47 for a pair (we can't get them individually)
Kind regards
Mark Hamson
Parts Advisor
Soper Of Lincoln
So there we have it..
If you want t to lower your car 30mm use Eibachs at just over £200
If you want to lower your car 15mm use OEM sports springs at just over £400
I am after the exact same thing, as I find the -30 from Eibach already too much.
They guy I order my Eibach springs from when I'm doing Zshed builds is £160 inc delivery. Any model
i paid £130 delivered from Germany for mine

Okay.. using @GazHyde gazza's sport models VIN l have just got reply from Sopers:
Hi Alan,
The prices for springs are:
Front springs - £83.48 each
Rear springs - £203.47 for a pair (we can't get them individually)
Kind regards
Mark Hamson
Parts Advisor
Soper Of Lincoln
So there we have it..
If you want t to lower your car 30mm use Eibachs at just over £200
If you want to lower your car 15mm use OEM sports springs at just over £400