For those with a Vauxhall Astra G (that's the version that went from late 1990's to mid 2000's - can't be bothered to look up exact year range - if you have an 8-valve version you'll be likely to have experienced engine thermostat failure by now. You'll also have been stunned to have been quoted a couple of hundred quid to have it replaced, because in their infinite wisdom, Vauxhall have built this engine around one of the cheapest and most unreliable components - the thermostat. It's buried so deep that the whole timing belt has to be removed in order to get to it, if you follow the official procedures, although a friend of mine with one did manage to wiggle his out by bending a few things back in order to avoid having to do that.
If your thermostat has failed open, in which case the engine never seems to warm up and will use more fuel, the most common solution is to get a hot thermostat from a Renault 5 - 89 degrees c is the hottest I've seen - like the one in the picture and insert it into the top hose. It is helpful to drill a small hole (1 - 1.5mm diameter) at the point where I'm pointing my knife, in order to allow both air bubbles and a small amount of water through to ensure that the wax bit of the thermostat (the pointy bit) gets up to the temperature reasonably promptly. It should be installed with the small hole at the top (the thermostat goes in the top hose horizontally with the pointy bit closest to the engine. This is almost as good as replacing the original Vauxhall thermostat, and will cost about £6 instead of £200+. For all you could ever possibly want to know about this, go to this thread at the vauxahll Owners Network.