In article , TheScullster
writes
Towards the middle the clay is grey/blue and wet
At the other end the clay is wet and sticky

I think you are on a hiding to nothing what ever you do
We have used the vitax clay breaker with moderate success on the areas
of the garden where there is just 'clay', but we haven't even tried to
use it on the areas where there is the sticky grey/blue clay stuff.
We also dig in about the same volume of compost and sharp sand to clay
and keep adding more compost every year. We have been here 11 years and
the first beds we tackled are just beginning to look like proper soil
)

The plot is in a north-east corner and gets sun from about mid day to 6pm

Presumably in the summer months??

So, it is seriously clayey, low and wet and doesn't get much sun?
I am sorry, but, if it were my garden, I would turn it into a bog garden
for shade loving plants (

However, we do have a FAQ on clay soil.

Improving Clay Soil by cormaic:
http://www.tmac.clara.co.uk/urgring/faqsoil1.htm
?subject=send soil

and one on shade loving plants

Plants for Shade by Kay Easton
http://www.tmac.clara.co.uk/urgring/faqshade.htm
?subject=send shade

BTW, mushroom compost is considered, by some, to be too alkaline to be
usable in large quantities in the garden since the majority of plants
like neutral to acid soil conditions.
--
Jane Ransom in Lancaster.
I won't respond to private emails that are on topic for urg
but if you need to email me for any other reason, put ransoms
at jandg dot demon dot co dot uk where you see