I agree Alain, however I have come across an article written a number of years ago by a UK amateur G4NJU who describes a pretty good but fairly simple solution. He uses the 15,625Hz TV line frequency which appears to be locked to a Standard. You may have seen the article. If not, he picks up a signal from a Scart socket on a TV or recorder and strips the line sync, also masking the half line pulses during frame blanking (we're talking PAL here!). A simple divide by two to present 7,812.5 Hz to one side of a XOR phase detector. The other side starts with a 10MHz xtal osc. also divided down by 10 and 10 to get standards of 10Mhz, 1 Mhz and 100Khz. Further division of 128 produces 7,812.5 Hz.
The phase comparator feeds a varactor across the xtal to keep the whole lot locked and a centre zero meter shows lock. It apparently compares with Droitwich at 1 part in 10 to the 9. Just 6 chips and a handful of components. I'll try it out in a few days

Kind regards

Adrian