Rose Nuts
If your problem is the rose nut(s) that secure the downpipes to the head(s) do not use too much force as you may, (will), end up stripping the threads on the alloy head.
You need to buy the correct spanner from Motorbins etc.
With the correct spanner move it a LITTLE left, (to undo) and then right, (to re-tighten), (you can tap it lightly with a hammer), and see what happens - if it moves follow this left and right procedure until it becomes free - If it does not un-tighten freely use a hacksaw blade or similar to cut groves, (two) into the nut so that you can carefully chisel the nut off or get to a stage where the spanner will take the nut off, (using the undo/re-tighten a little procedure). Then buy two new nuts and coat the threads with high temperature grease, (after cleaning the threads with a small copper wire brush) and tighten carefully until almost tight. Then start the engine and tighten until there is no “blowing” – that is as tight as they need to be.
Remove them once a year and re grease.
Downpipes stuck in the heads
If this is the problem, undo, (loosen), ALL fittings in the exhaust system, both sides. Then use a lump of wood to cushion the blow when you hit the curve on the downpipe forward to knock the pipe out of the head. It will not come first time but keep trying and eventually you will move it.
It is not usual that they stick……I have found this on a number of occasions – a sharpe knock is what is needed, but ALL the exhaust fittings need to be loose.