I think Rory has explained this.
If you put 1k more onto your existing 6k investment, you will have a current value of 3, and a cost of 7, and you will be able to make a gain of 4 before you start paying tax.
I'm not really familiar with Celtic freeway, but if it is invested in the same types of stocks as your other fund, and hence percentage growth will be the same in both then I think it would be advantageous to switch into the ISEQ20 fund.
Open to correction...
Ix.