Not another customs question...

[sherbertsugar.livejournal.com]

I know these show up here a lot, so i'm sorry to post something so dull, but as far as my searching/checking memories tells me, this hasn't been answered before.

Basically I have a big, big order from celga that's currently waiting to be shipped. Most of the items are bought second hand from auction sites and don't have price tags on them. I'd really like to mark down the value to avoid getting hit be customs *again*, but i'm worried they'll catch the parcel and make me pay even more for lying about the value. I've heard that they can do that. So, I want to know whether customs can only charge you extra if the items are tagged as being more expensive than the declared value, or if they can make their own assessment based on what the items look like they are worth. 
Also, what is generally a safe percentage to mark down a package by, 50%, 75%, 25%?
I'm in the UK so it's potentially a lot of money at stake and I would really like to give myself the best possible chance
of avoiding customs fees. 

Thanks in advance