Hi,
just an option I'll throw out there,
I also have an arrow trailer, but no annex, the zip is already sewn into the main tent and if your annex fitted my main tent (it should ??) i would be interested in your annex and you could then just buy an annex to suit the zip in your tent, just a long shot, depends on which bit of the zip they got wrong,
I did enquire via Dave Benson caravans on grand junction road about an annex for mine as this is where the previous owner bought my camper new from and they did recomend somewhere, can't remember but maybee worth a ring,
PM me if you want to take this any further,
I live in the NE suburbs less than 1/2 hrs drive from you
cheers
Garry