"On the subject of improving health of "developing" Nations, surely that will lead to more not less?"
Quite the reverse, if the Western world is anything to go by -- the Birth rate lowers with improving health and education, presumably because this gives women more to do than just child-bearing, though it's more complicated than that. So bringing health and better education to those nations with very high birth- and death-rates will help to stabilise the population in the long-term.
As to your other point:
" are you saying that because we in the UK will have little impact on it then we should not bother?"
I'm not really saying that at all. Or perhaps, I mean that efforts should be focused first in the places where any difference will be significant and actually worthwhile. The specific measure you are suggesting for example of cutting off child benefit for the third child and any after that is unlikely to discourage people having large families and is more likely to make things harder for the children than prevent them from being born in the first place. As has been stated earlier, the "lost income" would be around £13-£14 per child per week, a loss that is unlikely to frighten people into having fewer children. And anyway, for those large families there are over three times as many small ones, approximately 3.7 million families having just one child in 2012 statistics:
http://www.ons.gov.uk/ons/rel/family-demography/family-size/2012/family-size-rpt.html#tab-One-in-seven-families-with-children-had-three-or-more-children
The net effect is that population growth in the UK is rather more to do with immigration than large families exactly. Although part of the reason immigration is impacting our population, it has to be said, is that immigrants often have larger families on average.
Anyway, I'm not saying we shouldn't bother, but that we should focus our efforts in places where they can make a difference. And here, in the UK, is not really that place -- at least, not by targeting family budgets. The most obvious solution given that I said that UK population rise was mostly due to immigration would be strictly-enforced caps, but this merely shifts the problem from here to wherever else those people wishing to come to this country end up going instead. (Not that such caps shouldn't be implemented, but they wouldn't tackle this specific overpopulation problem.)
No, the best place to focus is in those countries who are contributing most to the global population growth, such as Africa and Central/ South America. Which brings me to your final point:
"Perhaps we should not send so much aid to save people unless they agree to some for of contraception?"
This approach might be worth exploring to some extent but with a great deal of care -- you can't rely on the people you need to pay attention to, or be aware of, the message you're trying to convey. It's all very well withholding money from Africa, but that's unlikely to lead to more contraception take-up in itself because you have to fight a whole cultural and religious attitude as well. Keeping the money means, to some extent, not spending it on health care and contraception and much-needed education facilities. It also means less money going to the wrong places, as does happen a lot, and that certainly
is worth addressing: if we are going to give money as aid to foreign countries, it ought to be used for aid, and a lot of that money just isn't. How we give Aid, and what it is spent on, are issues worth addressing and urgently.