How to open casing on switch of qualcast 1500w lawnmower?

Don't touch it! Electricity doesn't take idiots lightly.

Buy a new one
October 2016

Miles Tools & Machinery has exploded parts diagrams of Qualcast mowers - mtmc.co.uk

The switch assembly appears to be fixed on with two screws onto the push handle - from below.

It's very doubtful that you'll find a lose wire - more likely a failing switch or some other bad connection in the power cable - perhaps at the plug end.

Good Luck...
October 2016