It sounds like the 'Generator' might actually be the boiler? It's more and more common these days to have an outside boiler and its actually OK.
Sounds like there is nothing to be concerned about and its great its all been replaced recently.
Having an Oil boiler is just like Gas really. Its quick to heat up and from the householder point of view is controlled in the same way. I've never lived in a house with mains gas!
The only main differences you need to know are:-
. You are respobsible for making sure your oil tank doesn't run out. If it does you need to re-prime the boiler which depending on your level of ability might mean getting a plumber in - I have done it myself and its not that hard.
. The price of oil goes up and down. Keep an eye on the price and order when you see it low (summer time can be a little cheaper than winter.) Try www.boilerjuice.com to get an idea of the cost per litre.
. You annual service needs to be done by a heating engineer who works with oil. In rural areas there is no problem getting one.
So you have two gas bottles. I suspect they are both connected up at the same time. The idea being when one runs out it automatically switches to the other so you can then replace the empty one whilst you use the full one. Its easy to do and again bottled gas is available everywhere - prices vary so find your best stockist. You can get this delivered if you want but at a cost.
In summary, doesn't sound like there is anything to worry about.