Katymac, I you can go to homebase and have them cut pieces of MDF to fit the full width and about half the length of your cabinet. Then simply screw in a couple of screws (or use those shelf holder things if you're feeling fancy) and you've got a strong, cheap, effective shelf. And it doesn't have to be terrible looking, depending on your level of DIY skill and patience.
Most standard kitchen cupboards have a range of holes for differing shelf heights. Remove existing shelf and support and take them to Homebase or b&q, they will cut melamine to match and sell you the correct supports. All my wall cupboards came with a single shelf so I only could have 2 layers of china glasses whatever. Added a second shelf and have loads more space