Locating an underfloor water leak?

Hi! My central heating system is of the sealed pressurised type and I am losing about one bar of water pressure every three days. The plumber says it is probably a leak under the concrete floor and suggests using Radweld or similar to fix it. Can this be right? The pipework has been in place for nearly forty years and there has never been a problem before. The pressure relief valve and tank have been professionally checked and no problems found. Can anyone suggest how to locate an underfloor leak, there are obviously no visible suspect areas. Many thanks, David.