Comment: IMO you responded correctly, the Martinair crew did not. It would seem to me that the controller asking for an offset is just another form of radar control, and a valuable one at that. (The alternative, of course, to specify headings to fly which would amount to the same thing, but in a more workload demanding way, until clear of traffic.)
When not being directly radar controlled, where an offset is approved without ATC reference, it should be to the right.
Just my 2p worth.