Have you tried the Colman's Tuna Pasta Bake packet mix (looks like this:
http://www.ocado.com/catalog/images-full/25923 011_L.jpg )? I know it's cheating, but it's lovely!
I usually buy those really big pasta shells you can get in Tesco as they holds loads of the tuna and sauce inside. Cook your pasta whilst you make up the sauce in another saucepan. We use two packets of the sauce mix for extra flavour. Bung two tins of tuna in it, a tin of sweetcorn and mushrooms.
When you've mixed it all together, put some grated cheese and some broken-up plain crisps on the top. Put it in the oven until the cheese has melted. Yum-my!
(Sorry,
Im a BusyBee, I copied & pasted my answer from CB).
Hope this helps! :o)