[AM160]

I know of one company where Prospect/BECTU had to make clear that it was not involved and so its members should continue to work as normal, the other union's picket line is not aimed at them, but should refuse to cover for those who are on strike & the union would help back them up on that.

correct, however an individual may choose not to cross a picket line even if they are not affiliated with the striking union. 

It is technically secondary action and is illegal as the individuals union has no mandate for strike action however within the context of strike action at the BBC I've never heard heard of anyone having sanctions placed against them (other than loosing a days pay like all the others on strike) if they choose not to cross the picket line.