Hi Dale,
I found it difficult to do exactly what you want (combining two incompatible aims into one document):
- Sum the transactions that fall between the fiscal dates, and then
- Separate them again into calendar months.
How about a compromise? Try this.

I moved the Inventory Table from the Inventory Sheet to the Summary Sheet. I find it is easier to construct formulas when I can see all the tables at once. Also, it makes for an easier explanation in this forum.
Formulas in the Inventory Table
B2: IF(AND($A2≥UK Fiscal Year::$A$2,$A2≤UK Fiscal Year::$B$2),"Yes","No")
(and some visual aid with Conditional Highlighting 'Text is Yes').
H2: IF(AND($A2≥UK Fiscal Year::$A$2,$A2≤UK Fiscal Year::$B$2),G2,"")
Footer cell in Column H: SUM(H)
Formulas in the Summary Table
B2: Inventory::H$22
This will automatically adjust as you add or delete rows to the Inventory Table.
Footer cell in Column B: SUM(B)
Now move(Cut and Paste) the Inventory Table to the Inventory Sheet

The formulas in the Summary Table automatically adjust if necessary.

I hope that the UK Tax Office sees the sense in using whole months in the fiscal year!
Regards,
Ian.