BOAC -
Do I assume the 'DBI' is like an 'RBI'? and throughout the whole exercise P2 had a fully working IRS driven compass and P1 a working 'DBI', plus a standby 'E2B' type compass?
Yes, the DBI is essentially like an RBI.
All heading information with the exception of the standby compass is IRU fed.
The logic in the RJ is that under normal ops, a single failure of an IRU will not cause either pilot to lose all heading information and also allows quick verification that both IRU's are providing valid data.
IRU1 drives the LHS PFD / ND, and the RHS DBI, and IRU2 drives LHS DBI, RHS PFD / ND.
Transfer switches are fitted and used as directed by Abnormal checklists in the event of various system failures to restore data to the EFIS.