It would be easier & cheaper to replace it, the nibs on nightlatches invariably go after a while anyway. Parts for locks (with the exception of the most expensive ones) are like everything else these days, made to be replaced, not fixed.
Google "sas-locks-and-training.co.uk/" These guys are expensive, but very good!!! it's not just your usual monkey see monkey do type stuff and a lot of detail is covered. You also get 1 week "on the job training" with a locksmith at the end of it. You get taught business set up t...