brought some psp22 last night as the local tesco's was selling it cheap it's only a 50g pouch and says it makes 500ml but i seam to remember that lots of people make it weaker so is it enough to fill 2ltr bladder or do i need more??

Depends how you wanna do it, really.

I expect it will be 'isotonic' but that requires you mix it at the correct ratio.
If you don't, then it won't be.

If I am just out on a lng ride, then I use a weaker solution of energy powder than recommended as it's easier to stomach & getting the maximum amount of energy in to fuel you is not critical.
If I am doing an event, then I mix it as recommended by the manufacturer. You need to be careful with this that you don't then start mixing in energy gels and stuff.

There's a Torq nutrition guide that they will perhaps e-mail you that explains it far better than I just have.

There's about 23grams of carbohydrates in 50grams of PSP22. As you point out it is suppose to be mixed with 500ml or water. So 46grams in a litre.

Your body can only cope with around 1gram of carbohydrate per kg of body mass per hour. So if you are 70kg you wont be able to process more than 70grams an hour. How you get this 70grams is up to you but by watering down the psp (and consuming 1 litre per hour of fluid) you would get as little as 12 grams of carbs from your drink. Not much really.

This is more relevant for racing but for all day or intensive rides its important too. I find mixing drinks to the correct strength works for me. Others I know mix them somewhat weaker and get carbs from other sources – energy bars, gels, fig roles, lager 😉 etc