Hi Ewan!
Sorry to hear of your headset binding; that's no good!
I wonder of the lower seal might have come unshipped and is perhaps binding at the extremes. It does seem the FSA lower seals vary a bit in how tightly they fit, and it can be easy to have one come loose and wad up a bit if it doesn't go in quite straight, so I would check that outdoors or with a bright torch so you can easily pick out the black seal and see what's going on.
This next suggestion is "out there" a bit, but since it has happened to me in the past, I thought I'd suggest it: By any chance, is it a cable or some accessory binding, rather than the headset? Sometimes, a cable will catch unseen at the limits and cause a similar problem.
If nothing else, I would suggest removing the bearings and checking them by hand to see if they bind throughout their range of travel. If they do, sometimes setting them in 90° or 180° different from their original seating will extend their lives a bit.
You may wish to check your assembly against this:
http://www.thorncycles.co.uk/FAQ/FSA.pdfIf you find yourself in need of replacements, FSA make them available, but I found (here on the Yew Essay version of eBay) it was far cheaper to simply purchase another complete headset. I did that and put the bearings away as spares against the day I'd need them. As I recall, I paid only USD$24 for the lot.
I hope something here helps, with others' suggestions. Please let us know what SJSC say, and how it all comes out.
Best,
Dan.