i agree with you oggers this is a diy forum if you want to give it a go go for it, i would put the render beads up around the window making sure they are straight and level with the existing render and i would rule off of the beads i would also rule off of the existing render until everything is nice and flush i would board the ceiling first thoughWell I've got a couple of fire places to get right first and if I end up in a sticky mess I go to plan b.
Regarding render could I run some Batten and section the wall off say 2m between battens, making sure the battens are level and use that to work to?
I know it's hard and I know its not easy getting it level, but no ones going to die if I get it wrong. I'm not sending a man into space I'm putting some muck on a wall