Why has the value of my investments changed from the time when I sold them to the time I received the money in my bank account?

The price of assets at the exact moment you request to sell may change in value when the sale is actually processed and traded (which is when the value of your asset is finally determined).

For example, if you request to sell your assets on Monday after markets have closed based on £x value, on Tuesday when the market opens and the asset units are sold, the value might have increased or decreased in value because the asset value fluctuates frequently throughout the day.