Three months ago I purchased a Warwick Pro Series Thumb BO 4 string from my local music store and recently I've discovered a ridge that runs along the grain on the back of the neck. It appears the wood split slightly, the ridge is fairly long and is wide enough to feel in some parts. I've not noticed this before and I'm not sure if it may have skipped my eye at the time of purchase. Is this a structural issue for my bass? Will it develop further if not dealt with? Any advice would be greatly appreciated.
Yes - this is definitely an issue!
The neck of a bass guitar is under very high tension. Having a crack appear like that makes me think that either the truss rod has been massively overtightened, or this bass has been dropped badly.
My first thought would be to take it back to the shop and discuss it with them - if it was there when they sold it to you it should not have been sold. I don't think it's going to be repairable either - usually for this sort of thing you need to be able to use pressure to get glue deep into the crack, but I'd suspect that crack will be just about down to the truss rod, as Andy commented.