Ok so heres what happened:

ok basically the top of the my forks had a little bump (only a small one, and it doesnt seem to be dented, not obviously anyway) and now I cant get the headset nut thing to thread onto the top of the tube. Meaning my now pretty much finished fixie cant be used. So i'm hoping that someone might be able to offer some help.
Is there a way round this?

Why dont you show a picture of the 'bump' so we know what youre talking about?

Question:

Have you had this all set up before? Because it looks like the top nut will bottom out before getting to that spacer.

Get an LBS to chase the threads and you should probably be fine.

did you cut the steerer tube? if you did, the cut has to be pretty clean (perpendicular) and the threads need to be clean as well.

If the threads are only a little damaged at the end of the tube you can easily chase them by had with a small triangular file. Clean them up enough to get the nut started straight (it looks crossthreaded ad all get out in the photo) and you should be fine.

Between the above post and the race that is still on the fork, he should be able to clean up the threads.

metric thread file.