There's been a few posts in various other threads, Tesco, Amex, etc, however here's the jist.

Tesco sell prepaid Visa cards in two flavours. The fee free 3V and the fee paid "Gift Card"

You used to be able to buy 3V cards and use them for financial transactions, such as an NS&I savings account, and pay directly in. So buy for £25, pay £25 in, cash out eventually, and pay off Amex bill. Effectively manufacturing a £25 spend on your Amex.

If you bought £50 of 3V cards, you gained 150 bonus clubcard points also.

This no longer works, as 3V cards can't be used for financial transactions.

However!

The fee paid visa cards still can go through as a financial transaction, so buy a £50 card for £53.95, and pay in to said NS&I savings account.
This leaves you £3.95 cash down, but gain 150 Clubcard points - leaving you worse off by £2.45 at a maximum.

At the moment, there is also the fuel saver, 2p a litre off with £50 spend, and also occasionally a helping hand voucher for spending more than £40, which can be redeemed against your next purchase over £40 in Tesco.

A bit of a faff perhaps, but for me, it's an easy way to manufacture spend, for a relatively small outlay.

There's a lot of info over on Head for Points

Have fun!