Hi Gavski Will be fine to tile the window sill as long as you prep the sill first. I assume that the window sill is either glossed or varnished and this will result in a weak bond between the tile adhesive and sill. Using coarse grit paper, give the sill a good sanding on all surfaces. For an even better bond, score a criss cross pattern onto the sill using a chisel or similiar tool. Give another light sanding to remove any raised wood fibres after scoring. Remove all dust and wipe clean with a damp rag. Now ready for tiling.
Out of interest, how will you be finishing the front edge of the sill ?
Sorry for taking a w,hile to get back - thanks for the help. The bathroom is having floor to ceiling tiles, including around the windows, I will chisel the ledge on the sill back flush with the wall. The reason for not completely removing the sill is that it is recessed into the brickwork at either end, also it would make it lower than the other window that has been previously tiled. thanks again.