My blade wont stay down?

I have a Hayter Jubilee 48, the problem is the blade will not stay on the lowest setting it keeps popping up, I have cleaned it all up and oiled it but this seems to have made the mechinism looser and the problem worse.
I had this problem a couple of years ago and had it fixed by my local repair centre, but i wondered if there was something i could do my self as it is not cheap.