1991 Laurel Medalist - heater fan stuck on full blast

Hi there, I'm really enjoying the 6 cylinder diesel car, but I wonder does anybody have access to a manual? Recently the heater fan went into full speed mode and nothing I can do seems to stop it. It goes off with the ignition of course, but from ign key accessories through to engine running position it goes full speed.

I've tried every possible combination of buttons in the heater control panel.

Since buying the car a few weeks back I noticed there are no lights under the switches that route the air to windscreen/feet, etc, which makes me wonder if there is something dodgy there...

Thanks in advance...