This was precisly what I was asking that I couldn't get my head around. I don't think you understood from the way I worded it though. WHat I meant was do I either:
a) drain the sump, take it off and change the filter, put the sump back on and then flush ----- which meant I would have a new filter in there while flushing out the majority of the old oil and therefore old oil going in a nice new filter.
or
b) flush the system, drain the sump (which is where I meant I would be losing 2L of new oil), change the filter and then top up again. ----- I know this way I lose fresh oil but also this is the only way I can see not flushing the old oil through a new filter.