Thanks Dvader, it seems to be working now, not sure why it wasn't but thanks for the offer to help.
The model is a rusty roof using a single plane hence using the cullmode = 1 option.
PC - Windows 7 64bit, Intel i5, 12Gb Ram, 1TB WD Black, Evga Geforce 760gtx 2GB
Laptop - Windows 8.1 64 Bit, Core i7-3632QM processor, 8GB of RAM, 1Tb Hdd, Nvidia Geforce GT 740m