Thank you for your reply eternaltransit.
The ticket was purchased on a Business Skywards Saver fare (lowest possible fare) and I have used miles to upgrade the legs I could (I have been lucky in that rewards seats were available on all except the sector in question DXB>KUL, for once!).
I would of used miles to upgrade to F for this leg also, should it have been available.
Does this mean I will be denied access to the First Lounge?