Hiya! I've signed up here just to respond to this, haha. I had the same problem, I was so gutted when they disappeared! I'd listened to the oldest ones on there (I think from 2009?) up to mid 2013 and was looking forward to listening to all of them!
Anyway, in my search for the episodes I'd not listened to I found this: sites.google.com/site/sharethearchers/how-to-download-episodes They have a lot of episodes from I think from about 2009 to part way into 2014 when it seems like it ends, however, this should provide quite a lot of past episodes for you! The only slightly annoying thing is that they're daily episodes not omnibuses (omnibi sounds good too!) so there's more clicking through and you get to listen to the theme tune 6 times instead of once, haha, but I've actually found them quite quick to get through, you just need to keep a note of where you've got to.
Once you've got to the end of those, this site has the omnibus from Sept 2014 onwards: audioboom.com/channel/thearchersomnibus which is handy!
This leave a gap though from 15th June 2014 - 31st Aug 2014, the equivalent of 11 omnibuses, which is a bit annoying, but I'm hoping to find those somewhere else, if I do I shall let you know!
Also, in hopeful news, Adrian Beal's site (www.adrianbeal.com) does now say it's being rebuilt and does have a link of 'The Archers 2017' on there, though there's nothing behind the links as yet. I hope that means that the episodes will return, as it was such a well planned out site and such an easy way of listening that it'd be nice if it returned. Even if it doesn't though I hope the links above help you to carry on with listening!
I'm mainly glad to see that I'm not the only person to want to go back and listen to 400 odd omnibus episodes! I've become a proper Archers mega fan, haha. How long have you both been listening for?
Sam