I think ESR did the gasket for me, but TBH I cant see it making the smallest difference!
Fitting is pretty easy, only mistake I've heard people make is doing the bolts up too tight and stripping them. They torque up with hardly any pressure so most peoples torque wrench wont even register it. If in doubt do it hand tight, the heat cycling of the engine tightens it all up apparently.
Oh and don't trap the rubbery bit round the oil cap under it either.