i would have thought most would mate including the back street ones, my guess is its just a build up of crud on the rim, whenever i fit a tyre i always spend time to wire brush the inner rim as smooth as possible before fitting the tyre as you would be amazed at the amount of old rubber and flaky paint that builds up there then 99% of the time there is no need for sealent. then i would always over inflate, chuck it in the tyre bath, a quick check for bubbles if none job done
i cant see it costing more than a tenner mate, gotta be better than blowing the fucker up all the time