Recently the heater on my S takes a long time to get warm. It used to warm up within a few hundred yards but now it takes a few miles (unless I give it some welly). I was wondering if the stat had stuck open but the engine warms up as usual so I doubt it's that. Any ideas?

 

Either the water in the cooling system takes longer time to warm up due to the colder weather or you have bubbles in your cooling system and you need to bleed it

 

Or the valve letting the coolant into your heater matrix is stuck shut, common thing on other Hondas after not using the heating for a while

 

More than likely your first guess, the stat stuck slightly open and the cold not helping.With the car cold, take the rad cap off and start the car, ought to be no movement in the coolant at first then you'll see it start to "churn around". If it moves from start then as they say, "Roberts your Mothers Brother" new stat.
I had this on my old CRX, quick cheap fix.(as long as you don't buy the stat from Honda!)