I have a 1998 735 BMW E38 Battery went flat yesterday, so I followed the jump start procedure, car wont start. It has plenty of starter motor revolutions, cranks the engine fine!! Just wont fire??

check battery terminals sande clean and earth cable to engine clean and good contact >>>>>>>>>>alternatively jump start battery with jumper cables ,if still no response ,can only be starter motor solenoid fault/starter brushes

ONE POSSIBILITY IS THAT EITHER YOU HAVE A PROBLEM WITH BATTERY OR THE SERIOUS ONE YOUR STARTER MOTOR -SOLENOID IS NOT RESPONDING.YOU HAVE TO CALL AN EXPERT FOR REPAIR.

firstly try with the spare key, sometimes they become uncoded. Then if you have a voltmeter check the battery to make sure there is still a good level of power in it before getting the started motor cheked.

it might be the solinoid on your starter motor, try tapping it with a hammer (its ok it is safe to do so) as the ignition is turned at the same time, it might free it...just an idea

the "click" sounds like the solenoid on the starter motor is engaging but that the starter is not turning. So either the starter motor has failed or the charge in the battery is too low. Try a "jump" start....if makes no difference, check starter connections....if they all okay.....new starter time

you did not mention if you car is auto or manual trans but try to jump start with a good battery first, then if doesnt work, use a long metal bar or wood that can rich the starter motor and try to knock it gentle while someone starting the the engine. if the engine start, most likely your starter motor needs to be repleced. but if not then you have to check the starter relay and other things that related to starting system.

try to jump start with a good battery first then if doesn't make change try try to knock gentle the starter using a long bar or wood while someone cranking the ignition key, if the engine start,then your starter motor subject to be replace if not then you have to check you starter relay and starter power supply

Check the battery +12v power connection to the starter is in good order,
Check the ground connection between the battery negative and the engine/starter motor is robust and intact.
If no problem with above, replace the starter motor.

It sounds more like the alternator is bad. This will cause the battery to run down.

its possabile that the starter motor has had enough try another one and that may fix the problem