Overtime is usually available, but there is limited availability; you need at least 3 days off; you can request overtime for your 1st and last days off; it is a first come first served system (within your rank). Pay is quite good - you will be paid normal allowances + per hour overtime payment (approx £17-20 per hour). So you can see that it can be quite lucrative to get a long day's work on overtime i.e. Moscow or Athens.
Regarding part-time lists, I hear that part-time is much easier to acheive whilst on shorthaul at LHR (have no idea about LGW)....seniority on Worldwide is so high that should you decide to transfer to Jumbos you can kiss goodbye to getting part-time.....