I have worked out thanks to Seb's comments that the problem I have with the use of manage_delObjects is (I think) that I am passing in the title or id of the object to be deleted rather than the object itself.
Not quite... manage_delObjects takes a string which is the id of the object to be deleted. Your example <dtml-call "manage_delObjects('the_id')"> was passing a string 'the_id' to the method. I might have interpreted what you were trying to do wrongly, but I believe you actually wanted to pass the _value_ of the_id.
I want to put the object into the REQUEST so I can still access it from a different namespace - at the point I want to do the deletion, my target object is out of scope.
In fact you just have to put the correct string into the REQUEST and then make sure you have the object you're trying to reference in your current namespace, e.g. by using <dtml-with foo_namespace><dtml-var "manage_delObjects(the_id)"></dtml-with>. If you need more help, post the snippet you're working on again. seb.