the tension in the cable for the rear sprocket is fine but the rear shifter will only allow me to shift though the two largest sprockets, the shifter then will not push any more you can then release and do it again but only up and down though the bottom two gears, I have taken it apart as far as is easy to do and cleaned it no better, it doesn’t appear to be a issue with a jamming cable it seems to be in the shifter, please help?

Shifters are not really user serviceable, you can try filling it with GT85, otherwise it's generally a replacement required sadly.

I had this when I replaced a cable on an XT shifter, turns out I didn't seat the cable correctly in the shifter.

SRAM?

Indeed, SRAM?

I've got a rear shifter you can have for £15 inc P&P

snapped the internal mechanism of a Deore shifter last january bits of plastic grinding in the shifter, finaling jamming the shifter half way round skyline, what fun riding a two speed.

What shifter is it then?