Fresh or frozen is best. The added salt from the canned spinach might be causing his bottom-sitting. I use fresh spinach but frozen peas and zucchini. Gel food can be frozen and keeps for a while this way.
You can actually make your own gel food that is comparable to Mazuri. It seems like a lot of UK goldy keepers have trouble getting it. Proteins and fats are essential for growing goldfish. Diet is one of the largest factors in long-term goldfish health.
Gel Food Recipes
Here is a good cook book (so to speak) of goldfish gel food recipes. Some of them include Mazuri gel, some don't. Given what you can find locally, some might be easier to make than others.
Fruits I like to use are orange slices (mine go wild over them but this is a very messy treat) and some of the berries like blackberries and peeled blueberries. Just make sure you wash them to remove any remaining pesticide residue. Also make sure your hands are free of soap when handling their fruit.
Oh ya. I forgot to mention, the easiest way to feed goldfish gel food is to hand-feed it so you can monitor who gets what and the fastest doesn't get all the food. It might take a week or so, but most goldfish will eat from your hand. Plus, it's fun. ^-^