Item numbers,
11798 - Thorn special adjustable stem extension for 1 1/8in 28deg 170mm black
11797 - Thorn special adjustable stem extension for 1 1/8in 17deg 120mm black
11796 - Thorn special adjustable stem extension for 1 1/8in 17deg 170mm black
11795 - Thorn special adjustable stem extension for 1 1/8in 17deg 220mm black
Those are for the tube bits they cost £24.99
The best stem to fit is
0DEG30 30mm 0 deg Thorn CNC alloy ahead stem 1 1/8 in black £24.99
Though you can fit any stem to them.