Enclosures missing from feed XML after upgrading to WP 2.6

I’ve set an enclosure as a custom field. Previously (with WordPress 2.5.x) that would cause an enclosure tag to appear in the feed XML, however after upgrading to 2.6 the enclosure tag is missing. I’ve tried disabling all the plugins that are likely to interact with this feature but it does not seem to have had any effect – from what I can tell the fault began when I switched from 2.5.x to 2.6.

I really need those enclosures working again, my site contains mainly podcasts!

Thanks