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That 10mm play before engagement could be broken pawls,springs or if the bike is relavtively new it could be that the pawls or springs are stickying closed due to being over greased or cheap factory grease.
Most hubs can be disassemble to check the finer parts but as the other guy said £20 for a new free hub is a little less hassle if you aren't to familiar with disassembling hubs.
For piece of mind before you spend first gentlely heat the free hub with a hair dryer whilst free wheeling to see if the heat helps un stick any tough grease in the springs, i have success on a few customers bike as well as some new bikes that had a similar prob with this method