Havent you asked about the same issue previously?
Whilst the Omega is notorious for battery drain whilst not being used regularly, a battery should still be ok to start the car after a month at least (mine are anyway). Rather than condemning the battery straight away, if you haven't the facilities to check it yourself, I would initially get an auto electrician check the basics like the amount of drain from the battery whilst the engine is not running, and that no auxilaries are causing the loss of battery power whilst standing.
I would also get an independent drop test on the battery rather than from Halfords (which could be done by the auto electrician at the same time), which would confirm if the problem actually lies with the battery or not, and then you can look further into the charging system, and / or any other causes of the unusually high power drain.
As with most jobs, if you are unable, or don't have the equipment to do them yourself, you will save money in the long run getting a reputable person to do these checks for you, rather than 'asking the woman next door sons mother in law, who works somewhere that the cleaners mate knows someone who messes about with cars'.
A power drain on an Omega is common, but not as you describe, so pinpointing the actual problem, I would suggest, requires someone at the actual car with the testing ability rather than stabbing about in the dark with possible suggestions of causes that you may or may not be able to check yourself!
I don't want to sound blunt, but that is your best option and will be cheaper in the long run.