Brent,
Here is the sight I'd like to use on the miniature Martini but have never been able to find one. It's a Parker Hale Sportarget, I believe it is the model 10B. It appears that this sight was also made for a number of American rifles.
I found a PH 16 with a cobbled base and had planned to make a base like the one in the photo but then moved on to using the cleaning rod hole with the Lyman parts.
You are right about the BSA Model 8 and the PH target sights. They are hell bent for stout, tight fitted and precise. I wish the Model 8 was made smaller as the amount of elevation adjustment is excessive. It is a better choice than the Lymans for target applications.