My last one was one hours labour, and i tried a different bike shop that was quite close to me and got charged £60.
I believe my normal mechanic charges £50 an hour but i haven't been there for quite a while, and i'm in NW London.
Make sure you have the upgraded front sprocket nut too.
If you shine a torch onto the front sprocket nut, looking from the chain area, the old 9mm sprocket nut will not cover the shaft, so a few threads will be seen.
The new 12mm nut will be totally flush, so you won't see any exposed threads at all.
The nut only costs a few quid and needs to be changed for known safety issues, especially as the chain & sprockets are all being swapped anyway.