Elec shower - Light goes out when shower turned on?

This morning my Gainsborough 9.5e Electric shower stopped providing hot water.

The electric (h/w) switch (outside the bathroom) lights up, but goes out when the shower is turned on. When the light is on, it's not as bright as usual.

Is there a leak/short in my shower unit causing a safety power cut-out? Or is it an electric fault?

From reading other posts it seems I might have to get a new shower engine, but wanted to check this out before calling anyone out or sending off for parts.

Thanks!