Hi,
Just a thought. My previous Mig had the flashing spanner light, and the garage wanted silly dosh to change the main ECU, when others thought was it was just the key reader. However the garage had the tech 2, so controlled things. I ended up getting a complete set from a scrapper, ECU, Keyreader, and VIN and got the garage to fit and code a key. Cost a bit, about 100quid for the bits plus garage cost, but got me going.
So, it sounds like its the keyreader or whatever electronics that are intermittant/faulty. Its a bit strange, needing to get warmed up before working, but I don't understand the keyreader security system well enough to know what is going on. So assuming everything else is OK, eg battery, fuel supply, leads etc, a complete ECU/Security system replacement might be the easiest option.
Now, you might be able to get the complete set from a breaker, for example
http://www.ebay.co.uk/itm/Breaking-Spares-Vauxhall-Vectra-C-1-8-Facelift-2005-LPG-/270788934108?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3f0c4739dc I am sure if you offered him a reasonable fee he would sell you the whole system, which wouldn't need coding. You would need to get the VIN and Reg of the donor it came off so that if you needed a spare key, you could get a car pass. Thing is if the breaker assures you it all works, then you should have no problems.
I bought a new keyreader module for mine (= CIM ), it only cost about 65quid. Programming it all in is a bit tricky, but......
Ken