Just an update on this subject.
Having carried out the conversion to lower the seat, it left the car looking a bit untidy.
I noticed my E90 335D daily driver had a very neat plastic cover that hides all the seat mounting brackets. Why BMW chose not to fit the same cover to the 1M is a mystery.
I got the part number off the cover on my DD and purchased a new item to fit to the 1M. It required a couple of the mounting lugs modifying with a die grinder (dremmel). I then applied some good quality double sided tape to it and fixed it onto the Sparco mounting bracket I'd installed.
Here's before without the cover :
And here's with the cover installed :
Several posters contacted me asking how the seat belt/ pre-tensioner bracket was fabricated.
Here's a picture of the bracket we fabricated for the seat belt stalk/pretensioner attached to the Sparco side mount, (sorry about the picture quality) :
Note the pin circled in red, it's critical as it stops the pre-tensioner rotating in the event of an accident. It does not secure the bracket (there's a separate bolt that does that) it merely locates in the slot on the pre-tensioner bracket highlighted in yellow here :