This route is possibly one of the most difficult I've done from a navigation point of view. Very little in the way of signage specific to St Swithun's Way, and crosses with so many other named paths... although there are some stickers dotted around which only really serve to reassure that you're on the correct path. If you do this route, then I'd recommend to plan ahead properly so you know where to go and make sure you can recognise some of the sneaky turnings as you cannot trust that there will be signs to guide you - there won't be.
There's also a lot of road surface on this, so wouldn't describe it as a pure trail run. I ended up choosing road trainers to run this, and glad I did. No way would trail shoes have worked today, especially with the hard-packed dirt because of all the hot weather.
Hot day, so glad I took a lot of water with me! But did slow me down, so this time will definitely go when someone wants to tackle it :-).