recently had a slim line shower tray and pvc walls installed. however the plumber does not seemed to have bedded the shower tray in properly because when you use the shower the tray moves very slightly and a small gap appears between shower tray and pvc board appear. instead of ripping all out(at great Expense!) i was considering drilling up from the ceiling below and injecting some kind of material eg foam, cement etc to support the tray and prevent movement. ive heard suggestions of useing expanding foam but im not sure. any suggestions would be greatly appreciated.