Have you checked the boot light?

It all depends on your electrical knowledge, as to what you can do yourself.

You need an Ammeter or multimeter, and you need to connect this in line (Series), in other words between the battery earth lead and terminal, you can then see what draw you are getting, WAIT around 20-30 seconds before noting the reading, as when the connection is remade ECU'S etc will power up and it will take then a while for them to go back to 'sleep'.

If it's anything other than an expected 'parasitic drain' then remove fuses one by one until the reading drops then you can troubleshoot the problem circuit.

Martin