2000 Vauxhall Vectra dfficult to start engine

If battery o.k as you say , seems like something is draining it .This shows you how to check it for fault http://www.wikihow.com/Find-a-Parasitic-Battery-Drain

if you know the battery is ok,don`t nessecarily go by what halfords tell you,they could be wrong.you need to check the alternator,first make sure the alternator belt is not loose or damaged,make sure the wiring going to the alternator and starter motor is secure and good connection,then check the charge rate,it should be around about 14 volts with no electrical load,when you put electrical load on it it should`nt go any lower than 13.5 volts.if the reading is low then you prob need a new alternator,the starter motor could also be drawing too much current.you need to make sure all the wiring connections are good.

I take it was recently that the battery was tested ? If Halfords load tested your battery and said it was o.k I don't know why " 2145simon " should doubt that as it is a very simple test that every depot would have carried out many times and you cant really do wrong . Plus the fact , if there was any doubt they would have let you know hoping for a battery sale .The charging rate is also dependent on the charge state of the battery, and a fully charged battery will appose the charge more than a flat one.It is a simple test to check alternator output that Halfords or any other fast fit centre will carry out while you wait .The starter motor can also be tested to see what draw there is if suspect .