If you want to create a provision for a stock write down post a journal each month for the estimated write off value, debit P&L (Profit and Loss) credit balance sheet.
If you want to write off specific lines of stock, create a P/L account called "Stock write-off" Goods out the parts at full cost to the P/L account. Goods parts back in on P/L account at relevant value i.e. £1 each.
In Nominal Ledger create a nominal code to post the write off to.
Then go into P/L account and post a zero invoice picking up both debit and credit lines created by goods in and goods out. The system will create a write off for the difference, you can either leave the default as 18.104.22.168 or you can remove the nominal posting and select the nominal code above.