Q: is there a laid down requirement anywhere of what ground and flight training has/needs to be done for someone to move onto longranger ?
Yes in JAR-FCL 2.220 it states that even if a licence endorsement includes a type variant a differences or familiarisation check between variants is still required.
A diff check is generally technical course approved by the authority which can vary in length depending on the complexity (EFIS etc) of the variant and at least one flight hour (app1 FCL 2.261(b)) training with an flight instructor (FI or TRI).
As can be seen in 2.220 a diff check is required between all "L" variants (206L/L1/LIII etc) but not the "B" models!