Just bought a new bass a couple of days ago for a stupidly cheap price from some guy I know and it is pretty much perfect
However, the intonation on the B, E, A, D strings is slightly out so that the strings are slightly flat when I slide up 12 frets. Am I right in believing that to correct this I need to piss about with my saddles? If so, in which direction do I need to move them?

Also, the B string, as they tend to be, is a bit slack for my liking. I don't think there is anyway of tighting the string without changing the gauge of the string without having to change my action, is there?

If notes are flat move the saddle forward (ie toward the neck), if notes are sharp move the saddle back.
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Best to get it set up professionally. Then you can add heavier gauge strings at the same time.
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I can tell you now that piss won't help your intonation. However, it does involve the saddles. It's actually a pain in the ass because you have to keep tuning and untuning. It took me about an hour to get my bass's intonation set.