Classic Heater problems

Hi all, I have a problem with the heater of my 88 2.4 VM classic and I don't seem to be able to see the answer.

OK when the air con is selected with the l/h slider the fan operates as normal when the r/h slider is moved to the left to start the air flow. That I think is how it should be so all well and good there then.

If I want to just use the heater function the r/h slider has no effect whatever and I have no fan running at any speed selected.

I thought the whole air/heating system was similar to a Disco 1 I have, on the disco the fan motor overheated and welded itself to the housing, after replacement everything was OK again ( though by this time the air con was FUBARed for another reason so wasn't working anyway)

So what am I missing here ? is there a known fault/fix? or do I need to take a deep breath and get the whole ****ing dash out to have a look?

Anyone ?????

So is there anyone out there that has a clue as to what is wrong with my heating??

If I am being a complete plonker and have got my knickers completely twisted I don't mind being told that either.

I could do with some help on this, so any input will be appreciated. Thanks.