"some clubs draft in ringers (if it happens to be a 3rd XV of a club who's got teams in higher leagues for example)"
My local club (Marlow) play in Tier 7, the same league - Southern Counties North - as Chinnor III. Chinnor 1sts play in National 1. Both teams are unbeaten after 7 games and Chinnor have 35 pts to Marlow's 34.
I can't help but feel that they has to be some element of giving National 1 players, a run out in the 3rds
if they are returning from injuries at times!