Worked for a firm who used the Sylacryl backing and linseed oil for face putty, absolute rubbish way to do it in my experience. Sylacril takes ages to dry as does putty, the only advantage was the putty is easier to do neatly if you dust it off properly.
Repair care Dryseal is good but silly money, Hodgesons heritage is a bit cheaper, I use mostly Timbaglaze from chemfix. I know a place that use Stixall which is only a fiver a tube. I did a test with it and it works pretty well, I guess they are all pretty similar chemically. I find them good if you need to spray finish top coat after glazing ( which I try to avoid).
Fensa is not concerned with repair or restoration work.
There is certainly not enough building inspectors to physically inspect every window that gets installed.
I think its a bit like electrical work, technically you are not even supposed to change a socket or light switch but there is a bit about being a "competent person" somewhere in there, which means you can do it as long as you aren`t a total halfwit.
Ollie