Hols,
If you haven’t already got it then you could contact EH and they can supply you with the National SMR, most of which have 8 figure refs. However, I’m not sure what their licensing on the use of the data would be and also trying to match up SMR and TMA sites would be one hell of a job (unless TMA sites are tagged with their SMR record numbers somewhere behind the scenes). Doesn’t help for those places not on the SMR of course.
Just a thought.
-Chris