How can I remove cement stains from a newly fitted stone fireplace.?

The fitters had trouble fitting it and used lots of cement. the stain is between the backplate of the fireplace and one of the pillars at the side

try some clr and see if that works otherwise contact your local tile store and as them for some help
Try brick acid, you should be able to get it from your local diy or builder's merchants (UK)
a light solution of muratic acid and water would work. Just be sure to ware gloves and eye protection.