Thanks yeah. I am employed, but only very part-time and my earnings will fall in the the £102-£139 bracket.
Then you are getting full credit for the Basic State Pension because it is the Lower Earnings Limit of £102 per week that counts, not the point at which you actually start paying NI. Note that the LEL goes up to £107pw from 6 April 2012, and of course if your employer is breaking the law by not completing PAYE returns for you then you won't get the credit.
I thought the fact that I am getting child benefit for DS2 (8yo) meant that I get credits until he's 12 in any case?
Yes you do, so for the next 4 years you will be OK anyway.
Assuming that's so (and nothing else changes) I'll be around 4 years short of the 30 years. I'm 42, so have some time yet! Does that mean I either need to pay 4 years' voluntary contributions, or have a job where I pay NI for 4years between DS2 being 12 and my reaching age 65?
If you keep earning over the LEL you will be fine and the rules could easily change over the next 18 years, so don't do anything yet.
Should I need other benefits in the future, will that be affected by my having a break in contributions? (hadn't though of that shock )
You do need to keep earning more than the Lower Earnings Limit to qualify for Contributory Benefits, but purchased years or voluntary contributions only count towards pension anyway.