Hope I'm not too late.
I assume it's a flat felt roof. Here's how I sealed one off for a customer who was 'brassic lint' or skint, had no money a pensioner. Firt I laid a piece of string with a dowel /short strip of wood tied on one end. Then I rolled it in to a plug of roof felt as tight as I could. You end up with a plug of felt with string through the middle and dowel/ strip of wood on the end. Right coat the bottom half of the roll with cold lay bitumen and push the dowel through the hole in the roof follow with the plug[hang on to the string] When the plug of felt is reasonably level with the roof pull up on the string to pull the dowel up across the plug. This is a safe guard to stop the plug falling through the hole if to loose. Now lay cold lay bitumen in approx 9r" square around and over the plug cut felt to 8" square, Snip off the string then lay the felt on the bitumen gently tap the edges in to the bitumen and run a piece of wood around these edges lightly to smooth/level them. Job done. The one I did lasted for 15yrs before they decided to have the roof re-covered with fibre glass cover which is superior they say to felt. Mind you you may be able to claim off your house insurance if it was storm damage. Try but make sure you can cover the excess that may be on it ie the first £50 or so. Silicone does work but the surface must be perfectly dry for adhesion and it doesn't last very long before you'll be on it again, even if it's covered, I know l've tried it and was back on the roof in 6mths [my own this time].
bob, February 2007