Went about the task of adding third rail today.
I used British Finescale track for this as I've used it before with good results. The rail is simply super glued to the sleeper parallel to the existing rail.
It was a fiddly task, but I hit upon the idea of using Blu-tak style adhesive to hold the track in place while gluing and it seems to have worked well! The Blu-tak was removed after a couple of hours drying, and left no residue - except where it had mixed with super glue! This combined to make a nasty immovable gum, which I'm hoping to be able to remove with a stiff brush later on....
I think I did reasonably well with the third rail around point work, at a glance anyway it looks pretty good and after a test run with an EMU, there were no derailments.
I also tidied up missing sleepers around the joins in track work, using Peco replacement sleepers (I forget the serial number...) They are intended to slip over fishplates so have a large opening to accommodate these and the rail. In places where sleepers were missing but not near fishplates, I slid a thin piece of stiff card underneath the sleeper to raise it into contact with the rail and then applied watered down PVA to each, to hold in place prior to ballasting.