Surface fixing of a Plaque

Thanks for your question. The preferred solution for anchoring into masonry and natural stone would be a chemical anchor – the HY270 injection was developed specifically for masonry applications. Please check your local HILTI for availability with the performance of any anchoring system into stone being very much limited by the base material itself and given the huge variation in types and strengths it is impossible to test everyone.
The HY270 was tested in the largest range of base materials of any of our anchors but there is nothing that really matches the Portland Stone that you have; the thickness of the stone also needs to be considered. When you look at the published data, (attached), the minimum embedment tested is 50mm in a masonry unit not less than 100mm thick.
Due to the nature of the application suitability testing in the base material should be carried out to establish allowable resistances in line with your local requirements.