As above, you need penetrating oil. Plusgas is what you want, you can get it pretty much anywhere these days from your corner shop to Screwfix. The 3-in-1 will unlikely penetrate as although it's lightweight it's not going to get into the threads as easy as proper penetrating oil. The other usual suspects like WD40 won't penetrate properly either, it's not even an oil and is mostly naphtha which evaporates anyhow. Leave the Plusgas on for a hour, use as big a driver as you can get on the screw head, give it a gentle tap with a weight against the drivers head and then turn whilst gentle tapping it. Don't go over board on the weight behind the driver, cast iron obviously doesn't like heavy bangs.
If you can't get hold of Plusgas, cut up a small piece of candle wax, press into place over the screw heads and apply heat to the wax as directly as you can, i.e. flame or otherwise, just enough to melt the wax firmly onto the screw head. The wax will work its way into the threads. Not as good as Plusgas but does actually work in a pinch. The heat itself also assists in removing the screw. Again, as with the weighted driver approach, be sensible in the amount of heat you apply.
One quick question, what markings does it have on the casting? Not sure that is a type 14. Maybe wrong...(I typically am).
