Endometriosis is a common (around 1 in 10 women), non-cancerous condition where tissue similar to the womb lining is found growing elsewhere, most commonly inside the pelvis. Symptoms vary but can include intense pelvic pain and infertility. Endometriosis that is very deep and painful may need surgery, which risks damage to the tubes that drain urine from the kidneys to the bladder (ureters). To reduce this risk, surgeons may put tiny plastic tubes called stents inside the ureters. These stents can stay for up-to 4 weeks following surgery but can cause severe pain and blood in the urine. Squirting a dye into the ureters, rather than using stents, may cause less pain for women after surgery whilst not making the removal of endometriosis worse, the operation take longer, or increasing the rates of complications (such as bleeding or damage to internal organs). Before a full clinical study can be run, the investigators need to understand whether this is possible, by doing a feasibility study