Well, it was a mucky job but it's done now.
Removed the cone shaped nozzle from the RTV can and applied it directly to the inside frame. Then used 2 x 2p coins (taped together) to prop up the glass from beneath whilst the window was positioned. Screwed in the glass support brackets then got to work on trimming the RTV that was squeezing out the front.
Tip: to trim the RTV, try the following. I didn't learn this technique until half way through and made a right mess to begin with, wasting a lot of time in the process:
1) Use electrical tape to mask off the glass (masking tape won't stick), stuck about 3mm in from the the metal frame should do. If you don't mask off, when you run your licked finger across the RTV, it'll smear all over the glass, then you may have to use a combo of white spirit and a Stanley knife blade to painstakingly remove the excess, damaging your smooth beading in the process.
2) Once you've cleared one side of excess, immediately remove the electrical tape for that side, then carefully do one more sweep with licked finger (to fill the small gap that the depth of electrical tape caused), discharging any finger excess onto tissue paper/rag as you go in increments. The reason for demasking one side before starting the next is because the RTV will begin to set and getting a nice final finish will be impossible without another coating.
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