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Thanks for your question.
You've got to get it stamped on your new passport. You should send it to UKBA to get them to re-stamp it. You will have to send both your old and new passport (better send by registered post) so that they can note it and make the appropriate stamp and a fee will be chargeable, attach a covering letter explaining your situation. You will have to give them a telephone call about how much they charge.
It's conceivable that you could possibly be let back in to the UK in the method you describe but plainly it's not worth the risk, especially with the immigration policy being tightened right now.
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