Well it turns out the sewer was broken, which is not surprising as it is about 80 years old. Fortunately they don't have to dig up the pipes anymore, they insert an epoxy lining and then put a bladder in the pipe, to hold it in place while it sets. This did however mean that we could not any water down the pipes or flush the toilet, for 24 hours. After that we should never have another problem with it, plus it has a lifetime warranty.