Decent cable,pipe finder.

[shagster]

Checked my emails from when I ordered mine and it's identical do this: https://www.ebay.co.uk/itm/380474735528
(different seller, of course). 20mm x 10mm x 5.2mm hole. That 10Kg pull sounds about right. I wouldn't go any stronger than that or you'll find it a nuisance pulling things off it.

 

[RandomGrinch]

It's not just electrics to be avoided.
I've found plenty of hidden lead gas lighting pipes in my walls (and in pretty random places!). Even a cheap and nasty detector was useful in this case.