The clincher came from a blindside break by the Ripley scrumhalf and winger Lee Jones forced his way over in the corner.
ManchesterGrammar away will be a tough one for the Lancaster side in the fourthround.
An early try for Kevin McCrory and penalty from Mark Molloy put StAmbrose in the driving seat at Wirral Grammar.
The home side hit back butfoundered on some excellent defence.
With the elements in their favour StAmbrose went further ahead when No 8 Molloy scored in the corner and in thelast move of the game, captain and scrum half Matthew Casson dived over forhis try. St Ambrosze now travel to Bramhall.