Just as an update to this question, I submitted XML sitemaps directly to the blog articles and those pages are still not showing in the Google SERPs. It seems that new pages are discovered quite quickly (as per Google Alerts) but are then dropped from the index within a day or so.
The only pages which are returned consistently are links to the page which allows comments to be added.
The links which were initially identified as broken, were not actually broken so there was nothing to fix there.
Next step I can think of is to attempt some page sculpting by setting a noindex on the comments pages...
If anyone has any more thoughts or ideas, I'd appreciate your input