The point of roasting tomatoes is to blacken them slightly, which gives them that slightly caramelised flavour. This is absolutely not essential at all, it's a variation I invented which isn't in the original recipe, but I think it does improve the flavour a bit. Feel quite free to add the fresh tomatoes raw about 20 minutes before the end of cooking, they have a different effect which also lifts the flavour.

If you do roast them, halve or quarter the big ones, halve cherry tomatoes or even leave them whole, and stick them either into a 200 degree oven for 20 mins or so, or under a grill if you're happy to watch them. They'll explode a bit, but what you're looking for is the skins and/or pulp starting to turn brown. If you've left large tomatoes halved, chop them a bit smaller; scrape the whole resulting mess, liquid, seeds, browned skins and all, into the soup, and I usually swish a bit of water around the pan and add that too. The flavours you get from things like sugar browning are not replicable any other way, don't waste them!