We've done it a few times now, last time we lost 2, out of nearly 400 customers which we were well happy with.
We wrote a very short letter and spent the evening delivering them to each and everyone.
We didnt ask about it we just told them from the next clean the price will be £xx, and a little of the of the reason behind it, rising costs of everything etc etc.
And we didnt say any probs call us!! The price is the price
And as said already do it in the new year get the tips in first