I just read that vegetables change during the cooking process and then contain new and unrecognised chemicals.....

Of course this must be true!! If a stew's ingredients were just added to a communal pot and the cooking aspect is ignored, the Chef will soon be the major ingredient of the next pot!

I mean it is the chemical changes during cooking, which make the food edible.

Also, some foods are poisonus, indigestible etc. if they are not cooked!

Funrunna is right. You have to cook some things to make them digestible and palatable. If you overcook some veggies they lose their nutrition into the water. Stews are cooked for a long time to make them "blend" together.

My Mum was a serial veg murderer and now I always steam veggies and try to get them "el dente" (slightly undercooked). Potatoes can only be done el dente if they are new potatoes. A good tip for this is to boil them until underdone and then drain them and stuff a clean tea-towel over them. This causes them to do the final bit with their own steam. (much better for salad potatoes to be served cold).

Jacket potatoes need more cooking as they are usually "old" or stored potatoes.

i have also heard that tinned tomatoes which have obviously been cooked contain a chemical ,which arises during cooking, that is particularly good for preventing heart troubles.
I have tried to find out what ingtedient without success.
Perhaps someone else can confirm this.

As someone with a background in the sciences, and chemistry in particular, all this talk of 'chemicals' as though they were something inherently bad depresses me! Everything is made of 'chemicals' - you, me, the earth, the food we eat, the PC I'm sending this from, even water (eau naturale) is a chemical.
Any fruit or vegetable contains zillions of different chemical compounds contributing to it's structure, scent (and therefore flavour), colour, texture etc. Cooking causes changes in these compounds, breaking some down, building others up and re-combining yet more. Imagine how long it would take to analyse every trace of every different compound in your humble potato! I don't suppose anyone has ever been sufficiently interested, let alone been able to get the necessary funding, so it follows that a lot of the compounds are unrecognised or unidentified.
I can't remember the name of that magic tomato compound either but there is supposed to be even more of it in tomato paste - which figures as it is concentrated!