Aristoph,
we're on BA11/BA12 over Christmas so i've been looking as well.
according to www.flightstats.com for Oct 1st to Nov 30th the performance was as follows
BA11 43% on time
BA12 90%

BA15 67.5%
BA16 89.5%

The stats for December can be done on a day by day by flt basis but you'd need to do the sums.
Interesting that the outbound stats for both combinations are far worse than the inbound,might look at other routes etc to see if there is a trend?
good luck with yours.............Harry

EDIT...forgot BA15/16 stats include LHR>.SIN and SIN>>SYD and return(S)
BA15(LHR>>SIN 64%)........(BA16 SIN>>LHR 88%)