Vauxhall Insignia Heaters blowing only hot

Hi,
I have a Insignia Sri 2010. Yesterday the heaters started blowing only hot air. I turned the blowers down to the lowest temperature (lo) and turned the fans off. The heat coming through the vents was still hot. The dashboard got quite hot after a while and, not sure if this is coincidence or not, the radio would only turn up to a low volume. I'm no car expert by any means so am stumped. The engine temperature remained on normal.
Any help really appreciated.