Thanks all. It’s an historic organ that long predates the A=440 standard.
I was going to politely decline the invite (it’s only in a week or so, meaning I don’t have time to transpose and rearrange all the pedal charts). However, the string players are also very uncomfortable with this – especially the cellists with 19th century instruments which don’t have the tolerance for A458 tuning. I think the plan is either to hire a chamber organ, or ask the organist to transpose down a semitone and for us to play slightly flat – thanks to Charles-Nix above for the calculations!