Thanks to all who have given their excellent advice.
As said earlier, I was having mixed success out to 25yds but I had to change where I held my head which with my spec's pushed right into my face made shooting uncomfortable.
I must admit that when a 1" knockdown target goes over at 25yds with the open sights it's more satisfying than hitting a smaller target with a scope at 50yds.
Now that young Ian has volunteered to bring his peep sight along next weekend I'll see how I get along with that.