Bike wants to go left

The fork/bars whole headset sitch is straight (or as straight as i can tell by eye and under the mudguards). The headset turns freely in either direction and is very smooth. But the moment you start to ride the sensation is as if an invisible finger is pushing the bars on one side making the front wheel want to settle a couple of degrees out and causing a swerve to the left. It's noticable the whole time you're riding and makes no-handed riding impossible.

• First thought was, the way i've installed the brake cables is pushing on the bars. I've now ruled this out as the pressure only comes with forward movement. if you hold the bike level and pick it up the bars don't want to move. It's only when riding.

• Second thought was that the rear wheel was installed on the piss causing "crabbing". I checked and double checked that last night and re-seated it (in the rain and south easts with a borrowed spanner). It's straight.

• Third thing was that the front wheel wasn't seated properly. Checked this morning and it seems to be straight and true and seated properly.

• Fourth thing was perhaps the wheel/headset wasn't straight after all... treble checked it and re-aligned it again this morning. It is.

Set off for work and yep i'm still veering left.

I'm starting to think my frame and/or forks may be bent. I can't see anything obvious but i'm running out of alternative explanations.

After this thread and in 4 weeks you never took the wheel out?

Please tell me you did and it was an invisi-crack®