hi there not easy but doable 1.clean all the silicone and other materials from the repair site. 2. about 20mm each side of the cill force a wooden wedge gently to open up a larger gap under the window. 3. you must be able force a silicone nozzle as far into the space as possible (the silicon needs to go right under the window frame at the joint) 4. you need to use high quality silicon not the b&Q type see a window fitter for some natural cure type, pump in the silicon when you see it spread to the wedges stop take out the wedges and clean away any silicon that has been squeezed out , leave for two or three days. 5. you need a strip of upvc about 30mm wide not to thick as you will have to shape it round the cill this strip can be glued over the joint with super glue ( upvc bonds very easy with super glue ) this will take care of the outside. 6. inside make a hole on the cut joint put the silicon nozzle in and fill the cill till it ouzes out cut you should be able to tell when this is ok then glue a small strip over the split covering the hole. thats how i would do it
regards bob
r.machnik, December 2009
