Which compost to buy?

Hi everyone,

I've planted some hanging baskets, planters and have got quite a few things growing in the greenhouse. As a consequence I'm going through quite a lot of multipurpose compost. I was wondering whether anyone can recommend where to buy bags of compost from (preferably online and delivered) so it doesn't end up costing a fortune over time. But I also don't need 20 75L bags at once. So I'd like to buy in bulk, but not that much bulk 

Also, I've got spinach and chard and such growing in the greenhouse and want to see how long I can keep them going over winter. When I plant them into bigger pots (they're in trays at the moment) do I just use multipurpose compost again, or can I maybe mix it with some garden soil to stretch it out a bit?

And before you say it, I know that the cheapest way is to make my own    which I'm already doing but it's obviously going to take a while before I get usable compost. But when I do get to that point, do I just plant things up in the compost as it comes out of the compost bin, or do I need to mix it with anything again?

Thanks,
Juli

I have never used my home-made compost for seeds.  As well as in the garden I do use it for filling the bottom half or so of larger pots to save on the bought stuff. 

To use for seeds, I guess I would seive it then mix with some horticultural sand to aid drainage.  But unless it was steralised weed seeds will germinate as well.  I have heard of someone microwaving compost to steralise it but that doesn't appeal to me!

I used a mixture of old and new MP compost for seed sowing and potting on this year and I did microwave the old compost first to kill any diseases and weed seeds.

I put it in a microwavable bag, and nuked it in stages so it heated through evenly; leave the top open or make a hole in it to let the steam out - hot enough to steam gently but not glowing in the dark is what you should be aiming for.

You can do the same with home-made compost, but make sure you let any worms and insects escape first unless you have very unusual tastes in snack food!