Thanks for the advice all,and they are the stainless model.I hope you do use the stainless type in your photo. There are some plastic copies that will NOT support anything that pulls harder than a wee trout.
If anyone has those plastic ones fitted - change them!
Make sure there is something substantial supporting the back of the fitting position. The advice given already about stainless through bolts is sound.
That's a job I still have to do as my second hand boat has just screws holding good rod holders in place. Always another job to do - BOATS!