The British Gas SARs/Recordings will only prove what you already know. That you had the conversation with them and that they did not take over the second supply, so in essence yes the recordings will help. What (if anything) brtish gas do about the situation depends on your conversation with them, and posisbly consumer focus.

Poiwergen supplied the property prior to you moving in and would have done up until British Gas took supply (this will be 4 to 6 weeks after signing up with them). Hopefully there'll be a record of this with E.on, although my gut feeling tells me that IF it exsists it'll be damn near impossible to trace. This feeling is bourne from E.on only recognising your tenancy in 2009, so if a record exists its lying somewhere in your landlord's account which they're unlikely to check. ( I would ask them about this however).

In light of the tenancy arrangments its looking all the more likely that British Gas failed to completely act on your instruction and your Landlady took her time updating Powergen/E.on.

Finger's Crossed something else turns up.