A sealed system works on a pressure of 1.5 bar ( 20 lbs per sq inch) which is sufficient to blow out most sealants.
I am afraid the only solution is to re pipe thru the loft and dropping down to each rad. The cost of digging up all the concrete to find the leak is high and time consuming. I assume you have lifted all carpets and checked the floor with a dampness tester.
Concrete corrodes copper pipe so there may be leaks all over the system.
Sorry to be the bearer of bad news.
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