At least you have been looking (Thats what learning new stuff is all about!)
This site might help you:
http://www.techonthenet.com/sql/sum.php
But what you will need to do is put something like this into your query:
I havent actually tried this, but it makes sence. (Don't often use SUM function)
Code:
SELECT SUM(Referrals 2010) as "Total referrals"
, [Referrals 2010].Assess, Count(*)
AS [Number]
, [Referrals 2010].Time
FROM [Referrals 2010]
GROUP BY [Referrals 2010].Assess, [Referrals 2010].Time
HAVING ((([Referrals 2010].Assess)="No")
AND (([Referrals 2010].Time)
Between "07.00-07.59" And "19.00-19.59"));
I'm not 100% sure about what field you wish to count, so you can change that accordingly.
Good luck, and if you get stuck im sure someone will be able to help you out.
Rixxe.