Not too soon to be planting just make sure you keep the bags somewhere sunny and well watered. Not sure you need fertiliser, we've never used.

If you want potatoes bigger then marbles, they need more space. The bags are just too small. Try planting them in tubs, a wine barrel, or even and cleaned trash bin will give a more substantial yield.

I used organic potato fertiliser as they are greedy feeders and needs to be topped up regularly as the constant watering will quickly wash away any nutrients in your compost. That was a good mistake, by buying Maris Peer instead of Maris Piper as its a second early (as is Charlotte) rather than a main crop and although not a first early; better suited to being grown in bags in my opinion.

Agree Jimmy 2nd earlies better for growing in bags.

...or use a larger bag. Also too much fertilizer will compromise yield.

Are American bags the same size as UK potato bags?

I don't know I never use bags. They must have a limited growing capacity and seem very restrictive. All my potatoes are planted in the ground or those big storage tubs are pretty good. I'm from Irish stock, lol.

We used to grow all our potatoes in the ground now, with limited space for veg we have to grow in potatoes bags. As long as you start the potatoes off correctly in the bag and keep adding soil as the shaws appear, have in sunlight and water you'll get a reasonable crop. 2nd early potatoes are good as they don't grow as big as main crop and can be harvested that bit earlier.

Thanks everyone!