So if the paper goes out of vertical around a corner what's the best thing to do?
Should I butt the join at one end and overlap by a mm or so at the other to keep the next sheet vertical or to have a small gap and try and colour it in.
P.S. Have certainly found reading through the posts here very helpful. It has fleshed out info that I have found elsewhere on the web and being able to research the project beforehand gives you a much better chance of getting it right.
You should try to get a join just past the corner otherwise you will end up with wrinkles in the drop in the corner - if you must have vertical drops both sides of the corner you may have to overlap the join near the corner and this will be a variable amount. Try to use natural breaks like windows and doors to conceal difficult joins.
My advice would be to follow what stoneyboy has said and size and or cross line the walls. Because of the large pattern. Start from the centre of the chimney breast so you get a full pattern in the middle of the fire place (does not look right if the pattern is not centred here) and work from both sides cut all sections to go around the corners by 10 mm. If the walls are not plumb there is no way you will be able to match the pattern all the way down the corner, then use a spirit level when you come out or into a corner but in general you would always cut at corners allowing at least 10 mm to come around the next section of wall, except when you are papering around reveals you can keep the 10 mm edges stuck down better if you use some acrylic caulk smeared on the corners.