Jeremy Thorp (Letters, 4 June, page 34) may be interested to investigate the provisions of the Late Payment of Commercial Debts (Interest) Act 1998, which provides a statutory right to interest on late payments.
The act can apply when the contractual terms fail to provide a substantial remedy for late payment. The rate of interest provided for in the act is currently 8% above base rate, which is higher than the rate included in JCT1998.

Don't worry if the clause has been deleted from the contract – you will probably do better with the default mechanism of the act. Take a look at the Better Payment Practice Campaign website at for further information.