Thankfully the problem wasn't with the shifter; it was the cable. It was all frayed and had wrapped around itself, it was also pretty close to snapping. Guy in the LBS said he had never seen anything like it before. Cost me £100 for a new cable and bar tape. Felt like they took the **** cos they missed out on a £300 shifter sale.
That's the good news. The bad news is they said I need a new chainring, cassette and chain. So looking at around £300 after Christmas.
Said the chain should be replaced every 1,000 miles, which is every 6 weeks