Sram MRX shifters?
Easy to get the cable out of, just pull the rubber away from near the highest gear indicated on the shifter & you should see the nipple. Push cable through from t'other side & job's a good 'un
They have a small metal part that can come adrift, or the plastic inside may have snapped, both are common issues with the MRX shifter. The metal part can be re fitted, if you know where it came from but if owt's snapped internally they're shot.
They are easy to set up for indexing LGF, I've not had any indexing issues with them
That bike is pretty standard for a £150 BSO, used to sell that sort of stuff by the crate. Unfortunately selling them by the crate wasn't enough to keep the bike shop going even with the repairs that they generated!
£150 BSO? Thats Luxury that is - we have here the really really awful copies of copies which have 'clicks' and not much else. They are very stiff to operate and arent really suitable for anything. Unfortunatly standard fare on Halfords/ Tesco/ Argos/ Toys R Us/ The News of the World £39/ £49 specialsThe MRX are a delight in comparison -