Slipping cranks, drive chain slip?

Further to my last post

I've read through this lengthy post and am still getting the problem of slipping when hitting a hill hard. I'm using this wheel:

http://www.shopraleigh.co.uk/Product.aspx?pc=2&pt=262&pg=3069

The guy in my LBS won't accept it's slipping and I don't currently have the tools to break it apart and have a look myself - could anyone advise whether this thing has the right hub on? I'd imagined that anything sold as "flip flop" would have the correct threading on the fixed side.

Mr LBS said that because I'd run it with a freewheel and the fixed cog is brand new there might be some slippage from the combination of slightly (very slightly) worn chain and brand new cog and it would "settle in".

I'm taking it steady and run a brake but I do get unnerved when I feel the slipping when I'm hitting a hill (it only happens on really steep ones)

Apologies if this is covered elsewhere