With respect to our WW crew - the 100% contract crew tend to be rostered to the 900 hour limits, as indeed in the last year where many have had to be rostered repetetive standby duties to avoid conflict with the 900 hour limit.
This caused the WW shortages which resulted in the short term contracts - together with the EU directive (I believe) that required banked leave to be either taken or lost.
However, the average is brought down dramatically by the inefficient SH network, where 450-500 hours is the norm.
There being about 10,000 LH and 3,500 SH crew is where the 600 hours comes from.
What BA are/will achieve by lots more crew on part-time contracts, is the ability to achieve more than 450 hours from a 50% crew member - more like 550 hours. Same goes for the new mixed fleet - just look at LGW - mixed 777 and 737/319 flying gives close to 900 hours flying for all.
Be under no illusion - VR but more especially increased part time and reduced crewing levels (esp on SH) will result in dramatic productivity improvements in BA come next summer. BASSA have missed an opportunity to be involved in the future of BA cabin crew.