in my opinion welding is the only way to completely stop blowing/leaks etc most temporary fixes are exactly that exhaust putty goes hard and eventually falls off clamps etc have to be perfect fit and even then ive never seen any that work 100% so really i cant help you
good luck though
why not just weld it up and sell the exhaust with the car once you've finished with it or simply by a cheap standard exhaust at the time of sale 
Thanks for the reply I was planning to keep my standard exhaust anyway but if I get the reducer wielded on not sure how I would get my standard one back on.
How much as a rough guess would a garage charge to wield it up?
All the other joints are fine they just have a sleeve/clamp kind of thing so I was thinking of buying two more of them and see if they do a better job than the clamps that came with the reducer.