Popular UK sites for parents

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I need to run some cost-effective advertising, for people in the UK to take part in paid market research events and product/game testing. I have had some success with Gumtree, but now I need to specifically target parents. Does anyone have any recommendations for good UK portals, that have forums or discussion areas, or newsletters that can be sponsored, etc? I am sorry this is relevant to the UK only, I realise this is a very international setting here, but the events I am promoting are taking part in the UK. Thanks!