How do I know when yellow split peas are cooked? After having boiled for initially 10 minutes and then simmered for an hour my yellow split peas are still firm and a little gritty. 

If cooked long enough do they ever get soft like a red kidney bean?

They are the cheapest pulse on the shelf and I'm beginning to think the least responsive to cooking.

I soak these overnight so they are fully re-hydrated by the time of cooking.

I cook in enough water to cover them.
You probably haven't added enough water. Split peas are dried, cooking re-hydrates them, but only if there's enough to for them to absorb.  You need to add at least double the water as peas, a bit like rice in that respect. It's also possible you have old peas, or they are a variety that simply needs a longer cooking time. Hard water can also slow down cooking. 

Try using softened water if you can, and more of it. An overnight soak can also work wonders.