I've not had any problems with my Swift's seat post slipping but some time ago I replaced the Q/R skewers with Allen Key bolts as they looked neater and I didn't fold the bike very often. I had second thoughts and went back to using Q/Rs and found that the seat post was a tighter fit. I think when I was using the Allen Key bolts I must have over tightened the bottom one a little thus narrowing the the seat tube a fraction. The seat post is now a very neat fit and takes some jiggling to pull out of the frame - not much of a problem. I've checked the frame with a magnifying glass to make sure I haven't damaged the bolt holes or stressed the area around the seat tubes but all looks well. Maybe using a normal bolt instead of the Q/R may solve your problem - but be careful!