When were the BDC's re-Numbered into the CEPS system?
Pardon my ignorance

Though CEPS numbering was introduced in 1974, it seems to have been activated on an 'as and when' basis. Some BDCs continued with their pre-CEPS numbers, on the crane and in the formal records, into the 1980s.

There is an interesting discussion on this in the members' area of this forum at viewtopic.php?f=21&t=47&p=131#p131. There you'll also find a useful list of the numbers by which the BDCs were identified in official records at various spot-dates between 1976 and 1985. That list shows your Swanage crane recorded as ADS1561 in 1976 and ADRR95210 between Dec 1980 and Dec 1982.