It should be somewhere either on or near your property essbee. It will be rectangular. Dh has just popped in and said the first thing you should do is check with your neighbours either side of you, if they are having any problems, as it may be further along than you think. If they are having probs as well, then the blockage will be in the main drain, and not the toilet itself.
Where we live now, we have a cover right outside the bathroom, and where we lived before, it was in the back garden, but it may well be in a neighbours garden or in the street, so you will have to do some research.