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Five pound labour sound about right to be honest.
No need to mess about with a @ChainBreaker, fit pin in properly, ensure chain is perfectly tensioned with no binding and aligned correctly, and only take ten minutes.
Some people are happy to pay for that peace of mind, that if Evans does it correctly of course.
Edit - read that as £15 including chain, my mistake, TBH if chain is brought in shop, labour charge shouldn't be that much.
Evan Cycles put a 1 1/8 cog on instead of a 3/32 cog (they do exist).
They put an incompatible cog on your drivetrain.
If chain is slightly worn, best course of action is pay for a new chain but labour should be free (goodwill).