We suggest that you enter the stock valuation every month using Tools > Journals (although you may want to confirm this with your accountant/bookkeeper).

For example, say in August stock was worth £2,000; but was £5,000 at the end of September

Dr Inventory £2,000
Cr Cost of Sales £2,000

We suggest you date this on the 31st August.

On the 30th September create a journal to reverse out the August inventory balance:

Cr Inventory £2,000
Dr Cost of Sales £2,000

Finally, journal the September valuate:

Dr Inventory £5,000
Cr Cost of Sales £5,000

Both the latter would be dated 30th September.