best to get the wall plastered to a true finish, if thats not possible how about building a frame at the short end and putting a "shelf" at that end to lose the difference.

Or if theres 1½ inches of plaster(doubtful) you could set the bath into the wall. This would make it noticeable however. i think the shelf would be the easiest way to hide the out of squareness(?) as much as possible. 1½ is a lot to ask over a metre.

As Big All says, if you've got white tiles you wont notice it too much- by the way that is one hell of a gap!

You could always dot and dab one of the walls?

Thanks for that - this wall is causing me muuuch headache. I wanted to opt for the minimum of fuss possible when installing it, but that looks unlikely now. I was going to opt for a frame, then fill the gap with plasterboard and mosaic tiles, but would I need to treat that flat filled in area from the water running off the wall and sitting onto it. Normally water would run off a wall and directly onto the sealant and roll into the bath, but it wouldn't happen in this case, and i'm concerned about water turning this area green and mildew within months.

Kango the wall out and rebuild square.