I'd say put it on high for 3-4 hours, for a medium sized half-thawed roast, and you should be fine. The stock cube adds some nice flavour, but with salt and pepper, maybe a little seasoning salt, a bit of garlic and onion, you should be just fine. For liquid, you'll need the bit of margarine, and a little water. (Since yours is thawed part way, boiling water should not be necessary, cold is fine.) Water should be 3-4 cm in the bottom of the pot. You can do potatoes in the pot with the roast, but personally I like them better in the oven. Or try making a packet with tin foil to put the potatoes in, in the slow cooker, so that they keep their own flavour and colour.
Hope you have a great supper!