Levelboss and XTP mods ride extremely well. I'd go for one any time, in fact the other day i was wanting to buy a cracked XTP from my friend. It all depends on how expensive welding is in the UK. If you can get the frame welded properly, it shouldn't give you any trouble whatsoever. On the other hand, you might come across a grotty workshop which won't make a good job of it and so the frame will snap again. And finally, it depends on how you ride, if you're planning to do lots of drop gaps etc. or just ride normally. I'm sure it'd be much easier for us to judge if you posted up some pics of the cracks. A crack near the headtube isn't usually something to worry about, on the other hand cracked chainstays may be a problem.