A few crumbs of bread are likely to cause an immune reaction.
Sometimes coeliacs seem to be OK with one lot of gluten but get ill with a second dose if its quite close to the first.
Damage usually (but not always of course) repairs in 2 weeks to 3 months.
Addenbrookes is likely to call you up for an endoscopy after a year to see if you have been gluten free enough for you. My son ate GF oats, and products such as corn flakes with barley malt extract at a low enough level for coeliac uk's satisfaction and his one year follow up endoscopy showed complete villi healing.
Addenbrookes advice to him at diagnosis was to eat GF oats and barley malt extract of acceptable to Coeliac UK level so that the follow up endoscopy would tell him if he could eat them or not.