Fist of all a big thank you to everyone for their insightful input.
I'm in Stockport near Manchester (UK).
I feel I should explain that I kind of ambushed the people doing the concert, as the concert was taking place already, and I rang the person who organised it and said I would come down and record for free. I love recording, but don't get much action......I also don't claim to be an expert, but feel things like this are a really good way of learning (mostly by making the mistakes)
I obviously understand that to them, it's a performance and not a recording session, and I have kind of hijacked them.
I stated to them that for the performance, the limitations as to the mic stands etc might not get the best recording for them, and my hope is that they'll let me do an actual recording session proper after they have done this one.
I'll possibly be doing a rehearsal session first in which case, anything goes with regards to mic stands. I think I will keep it simple though ( although I don't want to go the mono route really). Like Hugh says, possibly the Rode NT1s left & right (high as they can go) and the SE X1A central, with possibly the SDC spot mics on the piano in stereo.
I'm looking forward to it, and if it sounds pants, then no one has really lost anything.