Yellow leaf on orchid

My orchid was given to me a year ago. I cut it down after flowering (probably not low enough) and it has produced 8 flowers during the past few weeks. A couple of immature blooms fell off soon after watering. I give the plant a soak in rainwater every 4-5 weeks and it has been fine until now (apart from a few white aphids - sprayed for those recently.) I put some orchid feed in the water last time.

The main large leaf has turned yellow. The orchid is kept on the windowsill in my north facing bedroom so plenty of light but no sun.

At present it seems quite dry. Roots at the bottom of the pot are white. It is planted in what appears to be wood chips. I know orchids suffer if overwatered so I do not water too often. I hope the photos help.