Title of a UK children's book?
Featuring "grey pumpkins". Invasion of body-snatchers-style plot
whereby x is on a quest to solve the grey pumpkin problem and finds
them eg in people's gardens. Age range probably around 11-13 years.
Long title?

Hello Fifi, and thanks for your question.  

I've got your story, I'm pretty sure, but it shows up as a television
show (in the UK, circa 1968) -- quite possibly there was a book of the
same name as well.  The description of it on the site below matches
your story line nicely, with some differences perhaps due to the
tricks of memory after the passage of a few decades.

http://www.tvchronicles.com/childrenstv/kidstvk.htm

The show is called, KNOCK THREE TIMES, and is described as:

"UK / London Weekend Television 1968 
A fairytale style serial about two young kids, Jack and Molly, being
caught up in a story involving a Goblin King trapped inside a Grey
Pumpkin but determined to make a bid for freedom."


This feels like the right story to me, but if not, feel free to get
back to me to let me know I should keep digging, or exercise your
Google-give right to reject this answer and try again.

Thanks.

Dave


searched on ["grey pumpkin" OR "gray pumpkin" story]

Amazon.co.uk lists two books with this title.  They are both out of
print, but they might be what you are looking for:

Knock Three Times by Marion St. John Webb
1994 edition: http://www.amazon.co.uk/exec/obidos/ASIN/1853261327/
1973 edition: http://www.amazon.co.uk/exec/obidos/ASIN/0854683577/


Karen