The spawns will always be smaller then aneus or paleatus spawns...but should get bigger then what you have now, as the fish mature. Try pulling the parents next time they spawn. I'm sure you have a spare tank (or two) sitting empty. On smaller spawns (and smaller eggs) sometimes it's easier to move the parents. I'll move the parents with extremely large spawns as well.
You have what I consider, the best catfish guy there is...in the UK. Look up Ian Fuller. He has been one of the featured speakers at OCA and the Catfish convention. There's a lot of clubs,auctions,and great fish people in the UK. Go to www.aquarank.com for some UK sites.