Grey square object made up of small square objects, moving through the sky at speed, daytime

As a child, having a family day out. Sitting on a west facing beach a square object approched, it appaered to be made up of squares all joined together. It appeared to be standard grey metal no flashing lights, just moving at speed.  The object moved in from the sea  and disapeared over some hills out of sight.  Even now many years later i still remember this clearley.  Some years later we moved to the area (1986) and occaisionaly drive past and think of that brief moving object.  I dont have any memory of this being followed by any planes.  Just a bright sunny day in Wales
Witness indicates that the date of the sighting is approximate.  PD