Why has the value of my investments changed from the time which I sold them to the time I received the money in my bank account?
The price of funds at the time which you requested to sell might/would have changed in value due to the small time lapse from you making the request to sell to when the sale is actually processed and traded (which is when the value of your funds is finally determined).
For example, if you request to sell your funds on the Monday based on £x value, on the Tuesday when the fund units are sold the value might have increased and decreased in value. The fund values are calculated on a daily basis.