Problematic Seatpost

Hello, I've been riding my fixed for about 6weeks now, and a problem has arisen in the last couple of weeks. My seatpost is slipping, making it uncomfortable to ride. The story starts when i was building the bike up.....

I originally measured the seat-tube diameter at 25mm on the nose, thought this was a bit rare, so took it to the LBS, where the bloke said that the frame had been clamped to the older seatpost so tight that the frame had altered, he readjusted to 25.4mm, gave me a cheap seatpost and was on my way, all sorted : )

But now the seatpost wants to move down into my seat-tube every time i ride it, no matter how hard i tighten the seatpost bolt. Took it Evans to see if they could fix it and they were too scared to touch it incase they damaged the frame, which has lead me to tell you about the frame's history. I think it may be weak in that area?

I've attached a clamp onto the seatpost to prevent it slipping down the frame, but now my problem is rotational shift, every time i go over a speedhump or turn a corner quickly the saddle moves about 30 degrees left & right....uncomfortable!

I don't know what to buy or what will fix it. Anybody have any advice?

Thanks

SORRY! Just realised i should have posted this in Mechanic's & Fixing.

I don't know what to buy or what will fix it.

A bigger sized seatpost.