The grout on my floor in the ensuite has started to crumble, Dose anyone know the best product for repairing the damaged patches. I don't want to pull it all up and re-tile.
Clean the surface that needs grout change then using a grout saw, remove the existing crumbly grout. Vacuum the debris to make sure everything is clean before adding the new grout. Prepare your grout mixture (best option for bathroom floor is SnapStone Urethane Flexible Grout) Using a margin float, apply grout on the floor. You want to make sure the gaps are being covered by pushing the grout mix in using a rubber grout float. Let the joints settle and dry before wiping it with a damp sponge. If you see any haze on your floor, just buff it off with a wet towel/cloth.
If the grout is crumbling because there is a problem with the grout itself, (like it was mixed improperly, etc.) you won't have to "re-tile" but I'm afraid, to fix it properly, you'll need to remove the old grout and regrout the floor.
Another possibility is that the original grout was not applied correctly (perhaps it wasn't fully worked into the spaces between the tiles maybe) and if that's the case, you can probably get by with removing the old grout and regrouting too.
On the other hand, if the grout is crumbling because the subfloor is flexing/moving you're just chasing your tail regrouting. You'll need to pull up the tile and grout and provide a more solid underlayment which will reduce/eliminate the flexing that is causing the grout to crumble.