A huge thank you to everyone for their help and guidance on this, I have now redesigned my original plan, both because of the feedback and in some way due to the none availability of steel stock.
I am going to start with 125x125x15mm base plate with 97.4mm did plinth bolted onto it from below, offset to one corner. I will see how it goes with just the clamping bolt to start with, if necessary I can always add some sort of locating pin at a later date.

