Neutradol Gel and PX17 is good, used it in a victorian property where despite ripping up the floorboards we couldn’t find the dead beast. We think it was in a small crawlspace between buildings, or possibly even next door. Left PX17 pots under the floorboards, and painted the Gel on the floorboard edges when we re-laid them and any other holes where the odour might seep through. Worked a treat, happy tenants.
Note: 17 posts in and nobody has actually suggested this?? It is common in pest control, we got our advice from the local authority pest controller.