I want to replace my tiny bathroom window with a bigger one. For some reason a previous owner of the house took out a taller window and fitted a short one so it will mainly be a case of removing infill. The existing softwood frame is 550mm wide but I can buy a standard 600mm wide UPVC window quite cheaply. I investigated the brickwork today to see if the lintel is long enough to do its job with the wider window. I found there is no lintel. The brickwork is rendered so I could put in a concrete lintel if necessary. I have a 4" x 4" reinforced concrete fencing post which I could cut in half to support the inner and outer brickwork of the 9" wall. Do I need to do this or is the window narrow enough not to need a lintel?

I'm no expert, but I'd say you always need a lintel otherwise the window frame is potentially carrying some of the load of the wall above it. Not recommended!