cancel the auto renewal. Buy your £10 Gbag, then if you wish, set up a renewal for the £10 bag.
Please see link below on how to change a recurring goodybag,
Turn off auto renewal, purchase the 10 pound goodybag which would be due to start when current one ends, when you have 50mb or 50 min left or less the option to start next goodybag early will show up.
Thanks all. Sorted now. Brilliant fast help. Also recently had very fast help from Monzo and they're a bank!