Here's something to think about Tony.
If you get a barlow and a wide of around 35/36/40mm (whatever is available) and a 15mm then along with your current eyepiece you will have
35/36/40mm
26mm
17/18/20 mm
15mm
13mm
and 7.5mm
Not a bad range really, plus using the barlow keeps good eyerelief, just loose a bit of brighness.
Alternatively a 40, a 32, and a 12 with a barlow
Gives 40, 32, 26, 20, 16, 13, 10, 12, 6. Eight for the price of 4.
Am I teasing you enought
As far as high power go, I've never used my 4mm and only used the 6mm once or twice. Seeing is just rarely good enough.
Seriously though, think about what it is you want to look at and select your eyepieces accordingly. Galaxies, clusters and Nebs wide field, generally low power. Planets, moon, double stars medium to high power, wide field is not necessarily as important.
I'd suggest make that decision first. If you're unsure, then one of each (wide and low 35-40, and narrower and medium12-15) and a barlow.
In the end its a personal choice. See if you can get a chance to try some out first before you buy. Maybe the shop you got your scope from would be willing to lend you a couple, especially seeing how much you spent there. Its called "after sales service"