Certanly in England the mains is a very accurate and reliable frequency source.
To use it I'd suggest an X2 rated capacitor (0.05uF?) in series with a largish (2 x 100k - 1W in series ) resistor driving two 4.7v zeners in parallel ( in case one fails - my 'intrinsic safety' background ) as suitable bit of signal conditioning.
Observe 4mm spacings between traces/wires.
Allan