Hello Emma and welcome aboard, this forum is the best place for helpful, friendly BSA advice.
A couple of things I'd like to ad to the previous replies.
Please google 'dry sump engines', the oil is stored in the oil tank and is pumped to the engine and then pumped back after lubricating the engine. When the motorcycle sits for a period of time the oil will drip into the sump of the engine and create problems when you start it. This is the reason for the drain plug in the alloy sump that Richard posted a picture of. It is much easier to remove the sump plug and drain the oil than remove the original steel sump plate, gauze filter and two gaskets.
To check the engine oil level in the tank it should only be done after running the engine for 10 minutes.
The spanners and sockets used on these machines are imperial sizes known as Whitworth and have fractional numbers. The letters are W, WW or BS. For example, 5/6" WW.
Good luck and I'm so pleased that you have taken on your fathers motorcycle.