I HAVE DONE IT!!!!!!
Thanks for all your help and useful pointers.
I now have BOG lunch booked for our first official day in MK - HOORAY!
I eventually found the 12 digit number at the end of the URL link for checking in online - I would've missed it if it wasn't for another poster who said that this is where the number was - it is a bit hidden otherwise and not somewhere you would think to look.
Anyway we are very happy now