I cant seem to put a new cable into my sram x0 twist shifter

It's SRAM. A simple web search for "sram mrx gripshift manual" led to...

http://bicycletutor.com/grip-shift-cable/

and

http://www.sram.com/_media/techdocs/SL_MRX_Classic_Ins_06_99.PDF

and

http://www.wrower.pl/instrukcje/SRAM/1999-DTM_MTB_E.pdf

You enter that question verbatim into a search engine and it will bring you to the same resources we would point toward.

SRAM.com has many resources, too. This one might work out.

http://www.sram.com/sites/default/files/techdocs/SL_3Comp_MRXComp_Ins_2_03_0.pdf

Maybe the SRAM website would have a manual.

Yup.

http://www.sram.com/sites/default/files/techdocs/SL_3Comp_MRXComp_Ins_2_03_0.pdf

http://www.parktool.com/blog/repair-help/shift-levers-shifters

I'd revisit the adjustments first before spending that kind of money. It sounds like cables have stretched and simply need to be readjusted, which may be normal.

http://www.parktool.com/blog/repair-help/categories/derailleur-systems

Ask yourself this... Why would the left shifter have anything to do with this and how do you know you'd get an 'improved' set on a new purchase?

Maybe look on eBay periodically for a deal or a stray right shifter.

Aren't boutique parts fun?

http://www.parktool.com/blog/repair-help/shift-levers-shifters

http://www.parktool.com/blog/repair-help/front-derailleur-adjustments

Might as well go Full Monte...

http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur

http://www.parktool.com/blog/repair-help/shift-levers-shifters

http://www.parktool.com/blog/repair-help/shift-levers-shifters

or

http://www.sram.com/en/service/avid/view.php?catID=1&subcatID=3

http://www.sram.com/sram/mountain/category/80

or contact them and ask.

You should be able to push the cable back on its self, as long as the shifter is set in the relaxed position. Then it will be a case of threading the new cable through the shifter housing.

took it to cycle mechanic who eventually succeeded ,basically it's just very fiddly he spent half an hour on it... thanks to staff at BIKEHUT Exeter.....

Stop everything! Do not take the shifter apart! Go to the rear derailleur and look where the cable is attached. There is a bolt with an allen key head, it is an 8mm. Losen off the bolt. REFER TO PARKTOOL.COM