Before commenting on method of crack repair, I would like a bit of clarification:

1) You say the crack is "across its length".  Do you mean the crack is generally horizontal and along the side of the beam or is it generally vertical on one side?

2) Where along the crack is it widest, 5mm?

3) The beam spans 6m, how wide and deep is it?  And how thick are the legs of the "U".

4) If the beam is "not vital to the structure", what is it for and why do you want to repair it?  

I haven't seen the beam myself, it has only been described to me. The crack is generally horizontal. I do not know where the crack is widest. The cross section of the beam is apparently around the same size as a piece of A$ paper, perhaps a bit larger. The beam is on top of columns supporting a valley in a roof. The valley fits into the U

*A4 paper

Sounds like it is a precast gutter beam, which is presumably lined with metal to carry the water.  In order to inject the crack with epoxy, you would have to get to the opposite side to seal it, otherwise the resin will just go on through.  If you determine that the crack is not structurally significant but still want to fill it, probably best to make it a bit wider and pack it with an epoxy mortar.

I am not sure if these images will work as they are hosted on our company web but if you would like to help me out but can't see them then let me know and i will e-mail you them.

No, I couldn't see the pictures.  You can try email alnarelle@hotmail.com