Hello,
I'm not sure if the UK version is different than US but if it's the same, keep this in mind.
P&L Report looks at All INCOME.
Sales Reports look only at Sales Transactions (invoices, sales receipts, etc.)
If you record a deposit and offset an income account, it will show on the P&L but NOT on any sales report because no sales transaction was used.
That might be why your 2 reports are showing different amounts.