SIR - The explosions of the oil refinery in Hemel Hempstead reminds me of the oil depot that was bombed by the German air force at Oreston near Plymouth during the Second World War.

It was not possible to reduce the flames so it was left to burn itself out, which took upwards of two weeks or longer.

This then became a beacon for the Germans as a guide to bomb Plymouth, which they did when they knew that a bomb dropped between the rivers Tamar and Plym would either hit Plymouth or Devonport.

N M DUNKLEY,

Worcester.