I totally agree Mallaig although other people don't seem to notice it.
What I usually do (depending on the recipe) is to add creme fraiche/soured cream near the end of cooking. Give it a go and see what you think.
It is sugar that you are supposed to use. I only found this out the other day. I have a new colleague at work and they are Italian and told me that you should always put a little sugar in with tinned tomatoes
Strangely, adding salt will actually kill a lot of the bitterness. The problem is that the quantity is quite critical, and will vary slightly depending on which make of tomatoes you use. You'd have to experiment.
If you would rather not use sugar I have actually done this today and I can recommend a few drops of Worcester Sauce. It really takes the bitterness away and is more healthier than sugar.