Just select a month and it shows you which configurations have been operated on specific flight numbers during the month you selected. If it's showing multiple configurations for your route, you can also look up exactly which registrations (and their configurations) operated a flight for the past few months to get a better idea of which configuration is most commonly used - and other things as well - in other search functions on that site.
Cobalt and B747, Thanks for the input. Looking on the site cobalt gave ( and thats my weekend booked up!!) there seems to be two. I was hoping that the JFK-LHR had the PE seats in the old Bus class area, but i had to book row 36!! Perhaps it is a default seat plan as the other plan has 36 PE compared to this having 30. so if a plane has to be changed there are 6 spare seats rather than upgrading/downgrading 6 PE. will watch the allocated planes with interest over the next 2 months! Best Wishes