Check out the guides section for cleaning the petrol guage sender units, bit of a fiddle though. Not heard issues regarding engine light coming on with faulty fuel gauge though.
A perfectly healthy battery should exhibit around 12.6 - 12.7 volts in an absolute ideal world. Charging voltage is about 14.4volts. Just as a matter of interest, why do you turn the lights on before you start the car? How long had you left the car before trying to start it this time?
Cooked breakfast, took the Zed out for a wee run, put a roller blind up in the guest room, filed a vat return, did some itunes library tidying, watched rugby, chill time now.