A Lone Parent with 4 dc (my situation) has an earnings disregard of £HOLD ON - You only get an earnings disregard for the first 3 dc - after that you get no disregard. BUMS.
So, my maximum personal earnings disregard would be £9,000pa. Anything over that will cut my UC by 65p in every £1 I earn. 
But that earnings disregard is lowered by 1.5 times the amount of housing support given within UC. So if I have £120.36 a week rent, that would be £6,258.72 pa. 1.5 times this is £9388.08.
Which would take me BELOW the minimum disregard limit - so my personal disregard would be the lowest possible level, the minimum disregard limit, which is £3,300pa for a lone parent with 3 or more dc. So anything I earn over £3,300pa will reduce my UC by 65p in every £1 I earn over that level. And it is done on Gross earnings, not Net. That works out as a disregard of 63.46 a week.
If I worked for 20hrs a week at NMW, I would earn £HOLD ON AGAIN. To earn the £212 a week after tax, or £228 before Tax & NI - A Lone Parent MUST work 37.5 hrs a week minimum?! Yet they are saying that a Lone parent can meet the amount earnt in 20 hrs?! That means that ANY Lone parent on NMW working for less than 37.5hrs will be subject to conditionality, surely? FFS.
I'll do it on 20hrs at min wage first...right, 20hrs at NMW is ON the tax & NI threshold, so I would assume that no Tax and NI is due. So earnings would be £121.60p/wk. Take away the first £63.46 which is my earnings disregard, and I am left with £58.14. 65% of this will be taken away from my UC, so my UC will be reduced by £37.79. Child Benefit IS included (i.e. the amount of Child benefit paid will be taken away from the £500/wk UC ceiling.), so my maximum entitlement to UC would be £439.50/wk after Chb is deducted.
£439.50 UC - £120.36 Rent (covered by HB presently)= £319.14 UC.
£319.14UC - £207.00 that I currently get from CTC/wk for 4 dc = £112.14UC
£112.14UC will be the MAXIMUM that I can get to cover what would have been both my WTC AND my childcare element of WTC.
Which, when you consider that right now, WTC for a lone parent working 20 hrs/wk is £74.42, that only leaves £37.72 . But THEN, before I take out childcare - I have to reduce my income by 65% for anything I earn over the disregard, which is £37.79. AAARggh! I will post this info in a clearer manner in a sec...