Ive got some g form knee pads and i think they are brilliant. However after about 8 months of usage they have stated to slip a lot. Is there anything else worth considering before i end up buying the same again?

If you’re looking for something as light and unobtrusive as the G-Forms then no I don’t think there are any others out there that are similar.
If you want to go for something bigger which offers more protection then there are lots of options:

Knee pad recommendations

dainese trail skins