For the gas cap, you'll probably find where the end of the cord extends over to the gas door mechanism.
Somewhere around the gas door...on the 'lock side' not hinge side, you'll find a 'thingy' that you should be able to pull/push/ etc and at least get you gas until you figure the rest out.
Wish I could help you with central locking, but I bet the shop would have an idea of what they did/didn't do...bet it's simple.
Upcoming comedy shows: May 20, Greenwich Village Comedy Club. 6 p.m. See Facebook for other shows and PM me if you can make one...love to see you there!
A recent set: https://youtu.be/995gLmVEVZk
Mangofy your E46!