I want to have an agreement for my ex to see the kids, he's for ever letting them down he promises them and me he'll have them and he never turns up.Now its getting to the stage where I don't even tell the kids he's having them cause they'll just get upset, now the kids even don't mention his name no more, I want him to have access to see his kids but he's messing them about, is there a way of going to a solictor's and coming to agreement with him what he'll have to sign

Yes, you can sign an agreement with him, but if he fails to take advantage of the visitation rights accorded to him in that agreement there is little you can do to force him to. You should also at the same time and perhaps in the same agreement deal with child support he has to pay you. If you get this embodied in a court order, of course, you can force him to pay that.

You cannot force him to be a father. You can set up a strict visitation schedule but not force him to adhere to it.

I'm not forcing him to see the kids, I'm opening the door for him to see his own kids and I think this is the best way I just don't want in 3 years time that he'll try for custody,I'm giving him every opportunity to see his kids,he says he wants to see the kids but he never turns up the csa for some reason can't track him down so can I ask in a agreement he must pay maintenance when he as them until he starts going through the csa

A visitation agreement and him not showing up has nothing to do with him being able to file for custody.

If/when he files he presents his argument and you present yours.

You cannot waive the children's rights to support by side agreement in MOST States.

Otherwise I don't understand what you are saying.

Don't you already have some sort of custody/visitation/support arrangement? If not, why not?