Where is your Phal positioned and how have you been watering it? They don't like being in a draft or getting strong direct sunlight.

It is away from the window and no draughts and had been living there quite happily. Hmm... I will move it just in case, thank you. I haven't changed the way I water them or anything else that I can think of.

As you "haven't changed the way you water them", I suspect over watering - at this time of year, they shouldn't need watering more than once a fortnight.
If it has been happy where it is then leave it where it is, orchids don't like being moved around - that even includes not turning it in the pot when you water it.

Urbanite-is overwatering soaking it in luke warm water once every 10 days for 30 minutes and then letting it drip dry before putting it back in its decorative pot? That's what I have always done.

30 mins is a long time to soak, I simply run water through until the roots look green and let drain before putting back in the cache pot.

I dunk individual orchids in a bucket of water unit the bubbles stop, pull it out, let the water run through back into the bucket and then let it drain off fully into the cover pot (proper orchid pots with a narrow base that allows water to collect below the plastic liner pot) or the gravel tray (depending on which room the plant is in). I take the bucket to the plant not the other way round, that way I know which way the plant was facing.
I also have a table full in the kitchen which share a gravel tray - I water them until the gravel is just covered (the pots have gravel in the bottom as well). These are the ones I inherited and I'm still nurturing them back to health after having to keep them in my cold, dark office at work for far too long.
There is generally some water in the pot/gravel to give a bit of humidity but at the moment they only get watered once a month - I'll start increasing that to once a fortnight in March.