Senior moments...
Will admit to not putting the oil filler cap back on after topping up the oil. Can't recall what caused the interuption in the process. I suspect I left the cap (and its dip stick) out of site.
Uneventful flight of an hour until was just short of destination when spotted fluctuating oil pressure.
Told "ATC" what I was experiencing and asked for immediate landing as a glide approach. Granted.
On the ground, most of the oil was sticking to the side of the aircraft.
The sun shines on the fortunate. Was able to borrow a replacement oil cap and, after a refill, flew home again. Found the bits of the old filler cap in the grass (perhaps it was the hanging about of the man mowing the grass that rushed me?). Drove the borrowed filler cap back straight away to the distant airport and ordered a new cap on the Monday which arrived later in the week.
Other members of the group have since admitted to similar "moments"...
Roger