All By Myself by Robert Scott

Seven years alone had driven Larry to the edge of insanity, but that's nothing compared to the shock of discovering he's not alone and never has been!

Quintessential Quintet - Frances Bartram

The Quintessential Quintet is an Enid Blyton parody.  Four children and their dog are out in the country for a picnic.  They encounter some odd flashes of light (which could be smugglers) and a "sort of" boy who may be a new chum or someone more sinister!