Climatronic on full heat

An odd one this morning.

Climatronic set to 21'c as usual and A/C off with the outside temp 10'c

When the engine got up to temperature, the heater just kept pumping out the heat.

I tried turning the temp dial down to absolute cold but it just kept getting hotter and hotter!

By th time I had driven the 10 miles to my destination I was a bit sweaty to say the least, despite having the windows open.

When I stopped, I had a play with the buttons and knobs and all appears well again.

I'm guessing it could have been a sticking diverted flap or a valve or something.

Any other ideas?