The 7366 squawk is a procedure whereby aircraft use this squawk to indicate that they are listening out on the MAN app freq and are not requesting a service and maintaining listening watch only.
It sounds like the controller passed traffic information to the contact squawking 7366 in a generic manner.
There's no blurring of lines - this is an approved procedure for this area - it means that less aircraft call a busy freq merely to get a FIS through the LLR. Some still call for a FIS - no problem.
HD is right in that in other circumstances you shouldn't leave a freq from which you're getting a service from without saying first.
But in this case, as the procedure is just a listening watch, so when you QSY - change your squawk ( as you're no longer listening out!) and that shows you've gone. You can call to say you're going en route, or not. The whole point of the procedure is to reduce RT. I imagine that you felt obliged to call as you had received traffic inf, but as you were not in receipt of any service you could have just QSY'd and changed squawk, however - if you're in any doubt - then call the freq.
BIG AL - in your case you were getting a service from BB, and attempted a courtesy call to Coventry who were not able to deal with you at the time - so yes go back to BB and, as I imagine you did, tell them you couldn't get hold of Coventry. That way - you've tried to do as you were asked and if it is vital that Coventry know of your position, then BB could telephone to pass traffic information
Bit long and rambly - but hope that is of use
louby