I don't read the manga and so I'm a bit behind the curve on what's going on in the manga, but if I understand correctly the negative Bleach chapters are ones set before the main timeline, a bit like Kakashi Gaiden. If so are they safe for a non manga reader to read? Or are there spoilers in them?

The negative chapters are one big Flashback, and NO they are not spoiler safe.

Yeah. This Flash back tells the story that caused the current chain of events, so in a sense the story is one big spoiler. Its starting to get REALLY good. Id go do some catching up. ^_^

The manga definately outshines the anime in my opinion, these fillers are doin my head in

There isn't anything between the anime's current postion and the last present-day manga chapter that will be spoiled (there isn't really anything to be spoiled, the anime is pretty close behind and nothing of much significance has happened)... so in that sense it's safe to read.

Under the circumstances, though, I'm not really sure why you'd want to read this but not read the rest of the manga.

I didn't know if this was important to the story line, or just filler manga like Kakashi Gaiden was. Call it natural curiosity.

Honestly, I'd say the negative chapters are more significant storyline wise than the remaining part of the Hueco Mundo arc.

Agreed, plus it shows background to the characters whom we had basically no insight on and needed more development. I like this section mainly because I love background information on big mangas so that it makes sense and such.