Great! She is seven next year so i'll carry on giving it to her.

I'm taking it in a blend for 'C' and blend for capilliaries, but is that enough for joint care?

Vitamin C is only one required nutrient.  Silica is still the most important nutrient when it comes to bones and joints.  You could add some nettle leaf for the silica as well as sulfur and other required nutrients.

I've read its great for osteoarthritis etc. I haven't got that but we all need to protect are joints as we age, don't we.

Yes, it is easier to prevent than to treat.

If a small dog can have 1/4 tsp 3 x day, what should we have?

Still 1/2 teaspoon three times daily before meals.  You will be getting additional C from your other herbs and foods.