[Laird Scooby]

Jim, there's a very high chance you have the correct gasket, also that the recess it should sit in on the head has become encrusted with a combination of old gasket, limescale and rust. Use a screwdriver to gently scrape round where the thermostat should sit and you will almost certainly find the recess is there. If so, clean it out with the screwdriver blade and smear some vaseline or silicone grease round the new 'stat seal after fitting it to the 'stat.