Having sat on the BALPA Flight Time Limitations Committee for a while last year, I can confirm that the reason the CAA allowed the weekly limits to be totted up the way they may be at present is that in the wake of the Bader Committee, when FTLs were first introduced (I think), the companies all pleaded that in the pre-computer era it would simply be too complicated to administer a 7 day rolling week for cumulative duty hours. (I don't know why they felt able to cope with a rolling 28 day month or 900 hours/year though!).
Anyway the CAA allowed the 'administrative' week that HugMonster now refers to.
My sources tell me though, that the CAA have finally woken up to the fact that all such records are now held on spreadsheets and databases with the automatic inclusion of any limitations into rostering programs and the automatic generation of warnings to rostering clerks when limits are being approached.
There really is no excuse now for not using a 'rolling' week and change IS apparently on the way!
BTW, CityFlyer have an administrative week from midnight on Thursday, and we regularly work 'seven days on' which straddle the changeover and exceed 55 hours in seven days by a considerable margin - at the moment it's all legal.
When the revolution comes......