Did you set an anti-theft alarm when parking the car the last time?
But I assume that it is something different.
Do you hear the fuel pumps running when ignition is on position 2 but engine still off, at least for about 5 seconds?
If not, check the fuel pump fuses and teh relays.
Have you checked all relays and fuses? Did you attach the battery correctly? Ot could you have damaged a main fuse?
If no fuel supply, no sparks.
Any possibility to bring the car to a shop for a diagnosis? Otherwise you have to test and I hope that you have a Bentley Service Manual.
Any other things which you noticed now?
First of all, check all fuses, also the ones under the drivers rear seat. I know that this a PITA with the electric rear seats, but with a bit of experice you can do it in 30 minutes to remove the seat and to attach the seat again.
If you do not know how, go to my website below and then to e38.org, there is a detailed instruction from David Cecil.