my rear mech has decided it doesnt like smallest 4 sprockets and wont change into these gears. Works ok on the largest cogs just wont go all the way down the cassette.

Full length cable outer, x9 mech.

Is my spring knackered, mech is only 6 months old? Anything else it could be?

Spring, cables or knackered shifter most likely.

When you disconnect the cable does it drop to the smallest cog OK?

some sram mechs have very tight pivots, and even the slightest hint of cable friction will cause the symptoms you describe.

clean the cable/housing (spray lube + little red hose jammed in the end of the housing)

take the mech off and see if it moves freely, if it doesn’t, there’s not much you can do about it (other than buy a new mech, which may or may not be any better/tighter still/have wobbly pivots instead)

cheers i didnt try disconnecting the cable, will investigate that and shifter at lunchtime.