seeing as vauxhall dont make a manifold gasket for the astra garret t15 turbo, i ws wondering is i use some of that gasket card that you buy in sheets and use some of that if thatll do the job or just burn up?
Get the surfaces ground flat (or DIY rubbed finish -- surface plate and valve cutting paste). Do it right and you'll not have to use any firegum, although a tiny smear is OK. Now would be a good time to address the issue of the bolts bending if you don't flat off the relevant parts of the turbo housing........