I recon that is a crack, however I would do 2 things:
1) Drop the forks out and double check severity inside the frame
2) Contact Wiggle about getting it resolved, sending them pictures showing as much details as possible and organising a suitable resolvement.
However, I would also continue riding the bike until point 2 has been resolved, just checking regulary. This could have been like this since the last time you cleaned it and you’ve been happily riding it. It isn’t going to explode causing impending doom. It might split and make the lower bearing a bit loose. Also you are not putting more than everyday use / strain on the bike so its not like it will suddenly fail when you land heavily after a back flip superman seat grab.