The VA is the product of the Volts and the AMPS that the battery will deliver ie power, but the important figure for a battery is the AmpHours. Bigger batteries have more capacity and can deliver the amps for longer -hence higher AHrs.
A diesel requires a larger battery than a petrol because the engine has glow plugs (heaters which are power hungry) and relies on compression only to ignite the mixture.
In colder weather the chemistry of the battery is such that the capacity reduces. This is why you always discover that your battery is gone in the middle of winter.
A small battery will generally operate OK in a diesel but will be exercised much harder. This may have some impact on the life of the battery but will be shown up when left for a long period with a small load (eg alarm or interior light) or when the weather gets cold. This is because it does not have sufficient capacity. This is a bit like trying to live on 1000 calories a day when you need 3000. You will manage for a while but eventually fade.
Also, if you do a lot of short runs, you will be taking large amounts of power from the battery and not running the alternator long enough to reachrge it so it will be limping along on less and less charge. You won't notice until it falls below a certain threshold.
This is also true if you are running lots of accessories.
In short, a battery designed for a petrol engine is insufficient for a diesel. Get yourself the right battery (specified as AmpHrs i.e how long it can deliver a specified number of amps)
With regard to the poor headlights, possibly the battery, but these are dependent on your alternator and regulator. IF your battery has little charge then the alternator voltage may be dragged down as the number of amps available are drawn by the battery charging.
I don't know what the specified current rating of the landy alternator is, but I would have thought it is quite beefy.
You could test this by measuring the voltage on the battery when it has been standing for a while -say overnight. This should be around 12.5 to 13 V. If not then your battery is not being charged by the alternator when the landy is running.
When the engine is running, the battery voltage should go up to about 14V (ie the alternator voltage). As I said, if your battery is dragging the alternator voltage down then it might not get up to this figure.
I would recommend getting the proper battery and then check that you get these readings. If not then your alternator/regulator/rectifier need looking at. ASk whoever you get the battery from to check these.
If you keep running on the ' ford focus' battery you will come unstuck when the weather gets cold.