Update:
I tried the heater again last night when the ambient temperature had dropped a few degrees to about 17 or 18C and it worked perfectly. Blasting very hot on HI and up to heat within a minute or two. Saved me a trip to the dealers. ?
I hope it's just the fact that when the ambient is 22C the car won't let me heat it any higher and not an intermittent fault.
Strange though, that I can get very hot air coming out set to HI, which I believe is a setting of about 34C (really 29C ?) when the ambient is 17C but not when the ambient is 22C.
Surely if it's set to 29C (real world), that is 7 degrees above ambient and should be pushing out warm or hot air but it wasn't.

At least I'm more confident it will work on those cold frosty mornings now.