Nick,
After RedRx failure probably nothing will work (stain heat-set)
Chemspec have a product, in their 'Rx' range, for fruit and dye stains which some times works on lucazade. It can be used spray/blot/extract first and if that fails you can use the same product via the thermal transfer method.
However I have encountered some lucazade stains which have proved totaly indelible - seems to have an affinity with some dyes.
John.