I have taken the Bank of England Base Rate history and applied it to your �20k, for the period 1st March 2002 to today:
01/03/02 07/02/03 4.00 �20,751.78
07/02/03 10/07/03 3.75 �21,077.98
10/07/03 06/11/03 3.50 �21,318.50
06/11/03 05/02/04 3.75 �21,517.82
05/02/04 06/05/04 4.00 �21,732.40
06/05/04 10/06/04 4.25 �21,820.97
10/06/04 05/08/04 4.50 �21,971.63
05/08/04 04/08/05 4.75 �23,012.42
04/08/05 07/09/06 4.50 �24,144.44
07/09/06 09/11/06 4.75 �24,342.39
09/11/06 11/01/07 5.00 �24,552.47
11/01/07 10/05/07 5.25 �24,972.72
10/05/07 05/07/07 5.50 �25,183.45
05/07/07 06/12/07 5.75 �25,794.41
06/12/07 07/02/08 5.50 �26,039.28
07/02/08 10/04/08 5.25 �26,275.24
10/04/08 08/10/08 5.00 �26,926.72
08/10/08 06/11/08 4.50 �27,022.99
06/11/08 04/12/08 3.00 �27,085.18
04/12/08 08/01/09 2.00 �27,137.12
08/01/09 05/02/09 1.50 �27,168.35
05/02/09 04/03/09 1.00 �27,188.45
It seems you would be due �7,188.45 (less tax, of course).
Interestingly, this is not unadjacent to factor30�s �back of the envelope� calculation using the 5% average. This is despite the Bank Rate being at 5% or more for less than two of the seven years.
The average rate of return over these years is 4.435%.