Over at The Continuum (now, due to the writing of Father Hart, my favourite blog) a discussion in the comments has generated a list of basic premises that ought to be held when dealing with the Church of Rome in ecumenical dialogue. I've laid them out in a mildly adapted way here and can only add my hearty agreement.
Father Lawrence Wells contributed the following points that ought to be essential:
1) Anglican orders are absolutely valid and always have been,
2) The papal claims of infallibility are rejected,
3) The Marian dogmas of Immaculate Conception and Assumption are lacking in true Catholic consensus,
4) The Reformational understanding of justification is the only correct reading of Scripture.
Father Hart rightly added:
When Rome understands our position, then we can talk. This is no arbitrary list. I would say that at least all of these points are necessary. I would add one more:
5) "Holy Scripture containeth all things necessary to salvation: so that whatsoever is not read therein, nor may be proved thereby, is not to be required of any man, that it should be believed as an article of the Faith, or be thought requisite or necessary to salvation."