I can't work out where the blue astra was from the description, beside, infront, slightly behind, who can tell..
I am sorry but unless I see the accident, I only have a 3rd hand chinese whisper... the person in the lane that is coming to an end has ultimate responsibility for making a safe maneuver and awareness of those cars around them, although it does sound like the blue astra driver might possibly have had ***** size issues

and been driving accordingly.
For all I know the op's car could have been behind him at the roundabout approach, changed lane to overtake during the roundabout and been caught short on the exit...
There are so many permutations of events leading up to the crash that it would be difficult to apportion blame from what we have. I am gonna go 100% with the insurance company saying wife of OP at fault.