Front focusing problem - sigma lens

[AFC]

I recently purchased a Sigma 18-300mm lens, however it appears to have a front focusing problem (?-focusing in front of the subject) at 300mm, I haven't noticed the problem at any other focal lengths yet (only used it once).
I've sent off for the free Sigma USB dock, however that won't come for 6-8 weeks. Should I take the lens back to the shop and get it replaced? Send it back to sigma? Or wait until the dock arrives and try fix it myself?
Will Sigma charge if I send it to them to get it sorted out?

[speedbird2639]

Assuming you are in the UK then under English Law your contract is with the dealer not the manufacturer. I would take it back to the shop and discuss it with them. Sigma may help you but they wld be entirely within their rights to refer you back to the shop.

[AFC]

Thanks for the quick reply