I'm posting this anonymously, for obvious reasons (if you guess who I am please keep it under your hat).

I recently finished my PhD and am about to publish the first paper from my thesis. No sooner than I start to make noises about it and publicizing the results than a senior person in my field starts ragging on the work, and claiming that it was all contained in work they did decades ago. Most of this work is in the form of seminar notes, and precious little is published. Promised books on the topic never appeared. When trying to discuss with the person in question where I think my work extends theirs, they come back and say 'I invented such and such to cope with that case' 30 years ago. On searching for the 'such and such', I find two old Comptes Rendus notes from the time which it turns out are not available from the BnF. Again they promise work that has since not appeared. Requests for relevant documents met with abrupt refusal.

I am not so much afraid for my career, such as it is, and I am keen to give due reference. Indeed, I don't care if all of my work was subsumed by this person's unpublished work - but it relies on more recent developments than the time periods generally referred to so I genuinely believe it is new work. The other person doesn't seem to agree.

How far should I bend in order to show I am not interested in 'covering up' or 'plagarizing' this person's work?

Also I would like to make this CW, but anonymously I don't have the power to do so.