Original electric underfloor heating mats in concrete (1970s build)

After having been told the devastating news by one electrician that the electric underfloor heating (installed when property was built in 1970) wasn't working, another electrician came today with a very different opinion. He reckons 3 out of 4 circuits on the 1st floor are fine (he didn't test the ground floor).

If this is the case, I am wondering whether it would be practical to replace the broken one in the way it was originally intended. The first electrician had said it was a lost cause trying to replace any of the original underfloor heating as it would cost around £20K to dig up the concrete.

However, a builder that came round today said it's unlikely that the heating mats were installed and buried in the concrete itself; rather it is more likely that they are between the screed and the concrete. He gave a quote of £1800 to take up and relay the screed on the two floors.

Does anyone know if there's any likelihood that the mats were in fact 'buried' in concrete? If so, would this method of underfloor heating have been any more effective (i.e. give out more heat) than putting the mats between the concrete and the screed?

The house is all electric as there is no gas supply to the cul de sac. Some of the houses have storage heaters but several seem to just have the electric UGH as the main heating system, so it must be fairly effective when fully operational.

Not sure if there is a way to get hold of the original design spec from 1970? Don't want to have the screed taken up for nothing.....