Formula to calculate sum of column range based on presence of value in another column

Hi all,

I've been trying for some time to setup a formula that sums a column range based on the presence of the value "NULL" in another column, which defines the row range.
I'm looking for the sum of Quantity (Column B) for cells in rows between NULL labels in Column A. The sum is to be captured in Column C, one row above the row with "NULL" value.
Problem is that the number of rows between "NULL" values varies, hence any formula I guess would need to be dynamic in some way.

See below example:

Example:

A B C
Value Quantity Sum
NULL 0
[time 1] 1
[time 2] 0
[time 3] 1
[time 4] 0 2
NULL 0
[time 5] 0
[time 6] 1
[time 7] 1
[time 8] 1
[time 9] 0 3
NULL
....
....


I've attached a sheet which might make my query easier to understand.

Any advice/help would be really appreciated!

Many thanks,

Allard

Brilliant Daddylonglegs! This works fine,many thanks

Thanks Daddylonglegs & XLAdept!