Strange problem from a new build

I finished building up a Saracen Zen that I bought off eBay with a combo of bits I had lying around the flat and more new bits than I intended!
Bottom bracket and cranks came with the bike.
As soon as I turned the pedals I felt a strange feeling in the rotation in the pedals. Best way to describe it is an ovalised feeling. First guess was that the crank arms were different lengths. They're not.
Tried to remove the cranks and BB but couldn't get them to move (not sure exactly how they should come off - Truvativ Howitzer cranks and presumably a Truvativ BB but it doesn't say).
Thought now is that there is a problem with the cranks and BB which are seized but wanted to see if anyone recognised the symptoms before attacking it with a big hammer!!
Oh yeah, the cranks will only spin a few times when upside down with no chain on and try spinning back when they get to a certain point before stopping.
Bottom bracket and cranks came with the bike.
As soon as I turned the pedals I felt a strange feeling in the rotation in the pedals. Best way to describe it is an ovalised feeling. First guess was that the crank arms were different lengths. They're not.
Tried to remove the cranks and BB but couldn't get them to move (not sure exactly how they should come off - Truvativ Howitzer cranks and presumably a Truvativ BB but it doesn't say).
Thought now is that there is a problem with the cranks and BB which are seized but wanted to see if anyone recognised the symptoms before attacking it with a big hammer!!
Oh yeah, the cranks will only spin a few times when upside down with no chain on and try spinning back when they get to a certain point before stopping.
Current steed is a '07 Carrera Banshee X
+ cheap road/commuting bike
+ cheap road/commuting bike
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arm removal is by self extracting bolts or ISIS/octalink crank puller. BB removal is by External BB tool.
Parktools :?:SheldonBrown
Those BBs don't look too expensive so once I get it removed I'll get it replaced.
+ cheap road/commuting bike
and only Howitzer BB are compatible with the cranks.
Parktools :?:SheldonBrown
Took the bike into Halfords to get the cranks removed as I don't have that tool. The second guy to look at it instantly says 'you do know the cranks aren't on straight?' they weren't - couldn't see that for obviousness!
Nice easy fix I think, whip the crank off and straighten it. Of course not! Extractor won't screw in as the thread is knackered. Needs to come off so they try anyway - threads stripped as expected.
Off to B&Q for a hacksaw to cut the crank off. Budget build was getting towards/past my top end before now I'd have been as well getting a new bike! Hopefully I can salvage cranks off one of the frames in my flat but I'm betting against it just now!!!
+ cheap road/commuting bike