Looking at http://www.firstcall-photographic.co.uk/userfiles/file/faultsonblackandwhitenegatives.pdf and http://www.olympusmicro.com/primer/photomicrography/bwprocessingerrors.html it could be one of the following things (or even combinations thereof):

• Underexposure
• Underdevelopment
• Inadequate fixing
• Variations in processing temperature (reticulation)

If I remember right, Delta 400 should not be quite that grainy, so if you are sure about having exposed correctly, reticulation might be a candidate, as this one increases grain size. These, however, are just some wild guesses. It's probably best to have a close look at the actual negatives and compare them with the list of faults as provided above.

Update: The above only addresses the low contrast/grainy look of the pictures. I somehow managed to completely miss the vertical marks... At first, I thought of streaks due to overagitation during processing, but they seem too regular. One cause of them might be a sticky shutter. So, there might be a problem with the camera, which might also be causing other exposure problems. Check if the vertical marks extend beyond the actual picture into the part where the sprocket holes are. If not, then it's most likely a shutter problem. If yes, it's not a shutter problem and it might be a processing problem (even though, as mentioned, it looks to regular to be streaks from an agitation problem...)