Potatoes in bags

I'm planting some first earlies in bags.  I have some shop bought manure which I was going to put in with the compost and am thinking about putting some Growmore in the bottom as well.  Is this OK and if I put this in what do I do about feeding later on - I'm not sure how long the growmore lasts.

Also, on a totally different subject, I was reading on one of the other pages that somebody has sown spring onion seeds in their greenhouse.  I have never had any luck growing spring onions, although everybody says they are the easiest thing to grow.  Have tried growing them for years without any success.  Can you start them off inside and then transfer outside when they are bigger and weather is better.   Thought this might be more successful!

Thanks, I'll give that a try and see what happens - it can only get better!