So, yes, indeed - it was the connecting wire that had popped off.
Not really sure about how to fix I pretty much dismantled most of the dash - when in actual fact I now think I only needed to take the glove box and radio surround out.
Quite easy really - all the trim pieces clip on so can be pulled out easy enough, then there were 8 ish screws holding the inner glove box in.
Once thats out you can get to the back of the heater controls, where I managed to re-locate the loop end of the wire.
Took a video of it once it was reconnected - although after this I did manage to bend the wire down a bit it wasn't as close to the top of the peg...
I had actually ordered a new control panel from Ebay only £25), thinking something had snapped off inside it.
Had I had to fit this the bottom console needs removing to give access to 2 screws holding the heater panel in - thankfully I didn't need to do this.
https://www.ebay.co.uk/itm/254865205686.