The message
from "david taylor" contains these words:

I grew onions for over 20years in Cheshire on loam in 2 locations.
Sunny position, pH about 6,moderate amount of manure, about 1oz/square
yard of growmore.
I had good crops and enough onions from 150 sets to last a family of 6
through to April/May from August.
I am trying to grow onions in Devon on a clay patch surrounded by
trees. I apply horse manure, pH is about 6-I dressed the vegetable
plot with lime last winter, I use growmore at the same level as that
in Cheshire.
A lot of the sets hardly grow at all and the largest onion is about 4
ounces. Some go to seed early, but much of the plot becomes shaded by
trees after the end of July.

I don't know whether it's significant but this year my onions ended up
on a patch which had had quite a bit of compost added over the winter.
They didn't like it. The soil seemed too loose and it took them a long
time to get a foothold. I resolved not to grow them on freshly composted
ground in future. I just wondered whether too much manure could have the
same effect. However, you used manure in Cheshire too, so perhaps not.
Janet G