National Ventilation 497 Condensation Trap 100-110mm with Overflow Connection

Although labelled for "100mm" ductwork, if it's grey, it's actually for the matching grey 110mm soil waste pipe. It's thus 110mm internal diameter, and 114mm external. You'd need a 3mm thick bush at each end to fit it to white 100mm ducting, as well as having a non-matching colour. It's clearly not fit for the purpose for which it's advertised.