Front wheel is slightly off, 4 or 5 mm over to the right. Is it as simple as loosening the bolt in the top cap of the headset, re-aligning the wheel, and re-tightening the bolt?

Hopefully this makes sense. Have tried google and previous threads, but can't find an answer. Thanks.

No – undo bolts holding stem a little, then slacken cap, then straighten, then tighten stem bolts, then tighten cap (not too tight – just right)

Brilliant, thank you.

Don't touch the top cap! (unless the headset's loose).

Hmmm.

I normally loosen cap, loosen stem, straighten, tighten cap, then tighten stem.

I thought if you tightened the stem bolts before you tightened the cap you might not be able to preload bearings in the headset and make sure it's all snug. Surely tightening cap bolt after the stem bolts will do bugger all?

DT78 is correct. The cap preloads the headset bearings and must be tightened first, then the stem should be alighned with the front wheel and the stem bolts tightened.