Before you try it, have a brainstorm as to if it could be anything else? For example the spare wheel not locked in properly?? Or something underneath the spare wheel, I'd suggest taking this out as a first port of call.
Its out of warranty unfortunately. These little black adjuster things? Do you mean the rubber bump stop things? I'd not thought of that so I'll give em a go mate. Cheers.
Yes, on the tail gate (not the car itself). You will notice that they have a hexaganol base to them so that a spanner can go round them. Just make sure you take your time and do it evenly. I would suggest reaching a point where the boot will not close properly then to just slacken both of them off just enough so that it closes.
But first I would just see if there is any movement in the bootlid. Try pushing the lid down when its in a closed position. If it needs adjustment I think you would see movement.