Hi Jacqui,
I haven't taken Xeloda, but wonder if your night sleeping might be improved by sleeping on your back and raised up on several pillows. That would help prevent the reflux that the Nexium and mylanta are trying to prevent. By sleeping on an incline, gravity will help keep the acid in your stomach and not your oesphagus, so hopefully preventing that horrible burning you are experiencing. I hate sleeping on my back, but have done so, almost sitting, due to reconstructive breast surgery, for the last 4 weeks. I have become use to it, though long for the day when I can sleep on my side again.
Love Chris xx