The greenhouse manufacturer provided 2" wood screws and rawl plugs which required a 7mm bit, so that is what I've used. I borrowed a SDS hammer drill with what I was told was an all purpose bit. This went through the metal flange into the 2' x 3' x 2" concrete flags with no bother at all. Although the galvanised screws have larger heads than the 7mm hole I think I'll get some washers under them to spread the load.
Thank you to all the good folk who have given me advice here on uk.d-i-y. I an well pleased with my greenhouse, now that it's up.
Don