Thanks for all your replies, I'll look further into what has been suggested.
The bike has all new cables and shifters, issue is at the 7th & 8th gear on the rear cassette, and the largest chainring.
Works fine in all 8 gears when the chainring is in the smallest & middle but will only go up to 6 on the largest chainring.
& stiffens and won't go any further in the front chainring.