How are you going to fit barrels to a Purdey with all the hand work that goes into making one of those? Maybe the current owner will get wind of this sale and reunite the barrels. Probably a long shot though.
Like many other shotguns that were built during a time of more hand fitting and labor, fitting a set of barrels from a different shotgun is possible, but dicey. If the frame size is the same, and the breech height and width is very close to the same, it could simply be a matter of removing a small amount of material from the hook and lumps, just as was done to fit the original set with stones, files, and a smoke lamp. If there is a loose fit of the donor barrels, then things become more difficult, or economically unfeasible. But metal can be added to the hook by TIG welding, just as in re-fitting a set of off the face barrels. This is why skeetx advised Mike to try to meet the seller for a fit test if he was interested in buying them. Cost of gunsmithing and fitting must be factored into the decision. Not a job for Billy-Bob, the scope mounting guy at the local gun shop. Purdey barrels probably aren't the place for a shim made out of a Pabst Blue Ribbon can.