GCT Discount question

I have been holding some shares with a particular company for 10 months, i'm 2 months away from being eligible for the 50% discount (i believe?)

my question is;
If i buy 1000 more shares in that company and sell 1000 next week for a quick profit will those thousand that I sell be classed by the Tax man as:

a) Coming out of my initial 10month old allotment and therefore impacting the amount i can claim the 50% discount on

or

b) the 1000 that i sell will be the same 1000 i bought a week ago and I pay tax as normal on any profit I make on those, but my other 10month old shares stay in tact?