The ground time in BKK varies, e.g.:
[KVS Availability Tool 5.4.7/Diamond - Amadeus Operational Info: BA 9/18 Jun 2010]
Code:
BRITISH AIRWAYS - BA 9
[Planned Flight Info]
LHR-BKK 22:00/Fri - 15:15/Sat 11:15
BKK-SYD 18:10/Sat - 06:05/Sun 08:55
[Flight Notes]
MEMBER OF ONEWORLD
LHR-BKK DEPARTS TERMINAL 3
BKK-SYD ARRIVES TERMINAL 1
9/ NON-SMOKING
ET/ ELECTRONIC TKT CANDIDATE
[Operational Info]
LHR 22:35 ESTIMATED TIME OF DEPARTURE
LHR 22:48 LEFT THE GATE
LHR 23:07 TOOK OFF
LHR - DELAY OPERATIONAL REASON
BKK 15:52 ESTIMATED TIME OF ARRIVAL
BKK 15:54 AIRCRAFT LANDED
BKK 16:00 ARRIVED
BKK 18:10 ESTIMATED TIME OF DEPARTURE
BKK 18:04 LEFT THE GATE
BKK 18:25 TOOK OFF
SYD 06:10 ESTIMATED TIME OF ARRIVAL
SYD 06:05 AIRCRAFT LANDED
SYD 06:09 ARRIVED
[KVS Availability Tool 5.4.7/Diamond - Amadeus Operational Info: BA 9/01 Jan 2011]
Code:
BRITISH AIRWAYS - BA 9
[Planned Flight Info]
LHR-BKK 21:35/Sat - 15:40/Sun 11:05
BKK-SYD 17:05/Sun - 06:05/Mon 09:00
[Flight Notes]
MEMBER OF ONEWORLD
LHR-BKK DEPARTS TERMINAL 3
BKK-SYD ARRIVES TERMINAL 1
9/ NON-SMOKING
ET/ ELECTRONIC TKT CANDIDATE
Ahh, so that is how BA fudge the difference between winter and summer time. With both the UK and NSW having daylight saving I think that there are 4 possibilites as one goes in and then the other goes out a week or two later.
LHR has a midnight curfew so I guess they want a bit of slack in case something goes wrong as they get the plane away, Sydney curfew finishes at 6am so no point arriving early. (simplified version - I know Qantas gets a few in earlier from 5:30am)