Hiya Lisa,
I had a similiar thing with my twins. They always coughed at night and more so when the heating was on.
We did buy an humidifier from healthy house, british allergy association and this did help and still does.
However, the coughing progressed to first thing in the morning.
I asked to be referred to a paed, which the doctor did. First thing they did was put them on a light dose of a brown inhaler and like a miracle the coughs vanished. They explained to us that they were astmatic but we had tests and it appears to be seasonal.
They were always getting chest infections and croup and we found since the meds, this has not been anything like it used to be. Their coughing started around the age of 2.
I always found that tonsillitis had a lot to play with the coughing too.
They can give a non steriod medication to children (just for info), it doesn't have to be a puffer, but the hospital have to prescribe it.
If you are still concerned ask to be referred to a paed.
Nic