I think we just underestimate how much good bacteria gravel holds. Plus when I re-filled my tank I did it with a hose straight from the tap but I left my filter sponges in so they were sat in clorinated water for a bit. That may have killed off a small amount of bacteria - not sure if that's possible but they were sat in tap water for maybe 5 minutes before I remembered to chuck in some stress coat. In hindsight I should've taken the sponges out before I stripped the tank but I'd run out of buckets so I left them in.
This re-cycling could well be my own stupid fault. Either way I am pretty miffed off about it but I just gotta keep going with the water changes and adding bacteria and hope it re-cycles soon
On a plus note - the cories are enjoying the daily fresh water! They're having a party at the moment