topsoil full of weeds

Sounds like the weed seeds were already in the soil, although some may of landed recently form elsewhere.
Its normally best to go for screened topsoil as this is relatively weed free and good quality.

Did the company mention it was weed free in any writting or on its web site? if so then you may be able to get something back from soil provided.
If it advertised it as just topsoil, then there was no reference to it being weed free, they were just advertising topsoil, so probably cant claim anything back.

Try removing some soil from the undisturbed area (trying not to mix surface soil in, and do a few sample across the bed) about 4-6 inches down and putting it in a pot and leaving it some where e.g conservatory (possible not a greenhouse as this is a bit open to elements) where NO seeds can land in it.
Weeds will grow anywhere even in soil in the house provided light and clean water can get to the sample. As its been undisturbed for 3 weeks, in theory if the soil is 'clean' then there shouldn't be any weed grow in your sample from below the surface. If you do get many weeds then it may be that the soil contained them already.

its prooving it realy . it`s worth definatly mentionining and never buy from them again and given the chance spread the word of the company.id of expected as nicky sais the odd one but if its full of them then it hasnt bean screened.id bight the bullet if they wont do anything and through some black polythene over them as they cant be to established yet.

Screened topsoil really only refers to the soil being graded to get unwelcome chunks of stone or other debris out of it. The only way to ensure that top soil is weed free is to buy sterilised topsoil however this also means that the helpful organisms are also killed off + it is really expensive.

As others have said, it would be difficult to prove if it wasn't said to be sterilised. I would cover it for at least a month, to exclude the light, ideally with black garden membrane. That should kill them off. Ideally leave it covered for winter, then remove it in Spring and plant it chock-a-block with plants. You could of course leave the membrane and cut X's to plant through it, then cover any exposed membrane with gravel or bark mulch. Or you can re-use membrane elsewhere if needed. Good luck.

Sterilised is what i meant not screened! I knew what i meant when writing it, just couldn't quite rememeber!

thanks all for the comments
the soil was supposed to be screened (basically sieved) and from a reputable local garden centre. I will try to get my money back.

..but if the garen centre has sold it as screened then as Ducky says, its not necessary going to be weed free. So you may not have any luck as they didnt state it was sterilised

perfect mad