Hi, I bought a set of second hand brandrup covers recently which were only 9 months old. They don't have the isofix labels sewn on them that are on the original seat at the isofix points but the base and back cushion covers are separate so don't obstruct the isofix fixings.
That said, I have read many times about the difficulty of accessing the isofix fixings on the bench seat and I can confirm that I've only tried once but gave up after about 10 mins (defeats the point of isofix a bit but I wasn't needing to use it, just trying install). The position of the cushions was putting the isofix arms at wrong angles for connection. I will try another day when I have nothing else to do and know that others have managed.
I'm not sure about the covers for the extra single seat, I don't have one for it, the front ones are base and back cushions and I would have thought it would be the same. As a plus point isofix is easy to access on the single seat it's just fitting the seat in the car
that takes the time there.
If you or anyone else has any tips on isofix fitting to the rear bench, I'd be delighted if you could share them.