Im pretty sure this is the pole set and specs.
http://195.238.172.57/acatalog/Polar-Vak_accessories.htmlAnd it says in the original link the hose is 51mm and mentions something about "51mm Side Inlet Cyclonic System" so a safe bet is its 51mm.
The inlet in that link is slightly different to mine. On mine the 51mm hose ending just plugs snugly right in the hole so yeh, its probably going to be 51mm.
One thing to add, two of the poles are currently stuck together. No damage just tough to slide apart. Ive wd40`d them so they could very well come loose with a person either end. I had a go on my own then wd40`d it. Everything else is immaculate. If i remember rightly its still got the plastic stuff on the trolley handle that was on it when it arrived. The packaging stuff. THats how little use its seen.