
IMO any type of sealant can be detected without any close scrutiny and as your windows are retained from the outside, any opportunist scrote with a sharp object could pop one of your DG units out without too much bother. As you are planning to replace your windows, what about retaining that sealed DG unit by carefully screwing into the frame and then covering with a smooth troweled finish to the mastic. Anything has to be better than leaving that window as it is without complete external beading. . . OR securing with wooden quadrant and sealant as a temporary measure.Thanks Scally Ho - sadly I have tried this and its a no - as Crank39 said I guess because its so aged no one stocks it - Madly, I wondered if you might be able to 3D print it?? I really don't want to have to buy a replacement window as its temporary whilst waiting for PP - Can you use sealant of any kind just to keep the windows appearing to be secure ?
Yes, and by using a narrow-gauge screw of a length to pass well into the frame, it will deter any opportunist scrote looking for vulnerable windows. By quadrant I mean a suitably small gauge of 1/4 round hard(ish) wood as the retaining beading. Pre drill the quadrant before fixing and you should not experience nay splitting from the screws.Thanks @Scally-Ho thats a really helpful suggestion - wooden quadrant ? and you mean screwing the beading into the frame?

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local