Seized seat post clamp: advice?

I've just acquired myself a Klein Mantra frame - to be precise, this one
URL:http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=3667944572
to experiment with. I've got a few wee problems I need to sort before I
can get it going.

The most serious is that the seat post clamp has an allen bolt rather
than a quick release, and at some point someone has used the wrong size
allen key and rounded off the flats. The bolt is also jammed and I
can't turn it. I'm tempted to take an angle grinder and slice through
the bolt, but I can't tell whether the seat tube clamp is integral with
the frame or not. If it is I would look bloody stupid because I would
then have a seized thread in there I would have no way of getting out.
Does anyone know for a fact whether the seat tube clamp on a Mantra is
integral?

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

;; Life would be much easier if I had the source code.