The clamp types are a pain. As said lubricant and twisting rocking pushing pulling etc. Will eventually free it.
Thanks for all your advice people, finally I have a nice new rust-free back box fitted and working... well almost. There is a small leak where the two pipes connect but a trip to the garage tomorrow will solve that. For those who want to know, this is what we did, I say we because a friend kindly did most of the work as I had never done this before. There is probably a better way to do this but we did not damage any part of the car. Just said a few bad words. Alfa Workshop (link below) supplied the exhaust clamps but the back-box was bought via eBay. Please unsure that you purchase the correct one as the tail-pipe diameters differ between petrol and diesel engines so I was told. Contact your supplier for details.
Parts & Tools used
1. New Alfa 147 back-box (http://www.ebay.co.uk/itm/1907323961...84.m1497.l2649
2. Two new rubber exhaust mounts (Courtesy of The Alfa Workshop, specialist Alfa Romeo garage and web resource
3. M8 60mm exhaust clamp
4. Small ratchet set with 15mm socket
6. Angle grinder
8. Large flat-heat screwdriver
9. Adjustable wrench (in case of wrong socket size)
10. Exhaust sealant
11. WD40 or other penetrating oil
1. Lift the rear of the car using ramps or jack stands (safely of course).
2. Use some WD40 or equivalent to try and loosen the exhaust clamp.
3. Remove the nut holding the clamp together (15mm) - We had to use an angle grinder to remove the clamp as it had been welded to the exhaust pipe (Argh!).
4. Once you have some new rubber mounts you can cut the old ones holding the back-box in place off, just make sure you support the weight of the back-box so there's no strain on the mid-pipe.
5. Now for the hard bit, you need to pry the flared end of the exhaust pipe from the mid-pipe. Just be careful you don't damage the mid-pipe, unless of course you're replacing this too. Prying with a screwdriver and some gentle persuasion from a hammer was enough but it was well and truly rusted on there. A buddy will really help here as they can twist the exhaust from the rear of the car whilst you can support the mid-pipe. Much tugging, twisting (giggity) and patience required, and perhaps a strong coffee because we were still at this stage after an hour.
6. Once loose remove the old back box. Next, we found it easier using a little silicon spray to help attach the new rubber mounts to the brackets on the car, it is a VERY snug fit otherwise. Just remember not to use penetrating oil which can over time degrade the rubber. Also, you may apply some exhaust sealant for good measure.
7. It seemed to be easier mounting the back box into place first and then connecting it to the mid-pipe afterwards as there is enough play in the rubber mounts to do so. Feed the back box pipe past the heat plates so you can attach it to the mounts. The mount near the fuel tank is easy but the other is up under the bumper, and difficult to gain any purchase on it. Again, a buddy will be a massive help. From under the car I could push on the bracket while a friend pushed in the opposite direction on the back-box.
8. Coat the pipe ends with Firegum or equivalent exhaust sealant. (Thanks to sprint veloce
for pointing this out)
9. From here you can feed the end of the mid-pipe to the back box and this is where a buddy is really handy. Once it's mounted properly you can feed the two pipes together, you WILL need to use some force.
10. Finally you can secure the exhaust clamp in place, a 60mm is fine as the original back box had a welded clamp type connection. Afterwards start up the engine, cover the exhaust briefly with a rag and listen for leaks. Annoyingly, we had one.
As you can see the old box was in a bit of a state and looked even worse when it was on the car. Annoyingly the old clamp was welded to the pipe (see pictures) as was the exhaust trim which I was hoping to save. No big loss though. As I mentioned I'm not saying in any way that is the correct way to carry out this repair, it just worked for us and I hope this is of help to others with the same problem. Thanks again people.