CC,
The car park for crew is located on the north side of LHR, by the Park Inn hotel, just off the bath road.
If you time is right, you can usually get into the crew room and report for duty in about 13 mins, from the time you have hopped on the bus. The crew room is now located airside, so you have to go through security before you report, so depending on how busy security is will depend of time of course.
Oh, and days off for the A320 guys is representative of what Lex has posted. Mind you in February I was rostered 7 days off before my 2 week leave, so had most of the month off
.
All the best.