OK Worked out the fix.
Make note of Static and Dynamic plan settings.
Create new plans called Temp & Temp2.
Master Planning Parameters - Change current static plan to temp - Change current dynamic plan to temp2.
Delete Static and Dynamic
Create new Static and Dynamic plans (Setup - plans - master plans).
Master Planning Parameters - Change current static plan to Static - Change current dynamic plan to Dynamic.
ReqPlanVersion then gets populated. Looks as though we had an oddball issue, perhaps created from setting up r2 in an incorrect order or (possibly) our reseller migrating this table?
Just to clarify - we have 2 master plans currently setup simply called Master and Static.