04 megane battery being drained in a few hours

Iv recently bought a megane 2004 1.5 Dci 80bhp 5dr
Hi Iv just bought a megane 1.5 dci 80bhp 5 door it's not been used for a while so I fully charged a good battery then fitted it when I collected the car an I then drove 65 miles on the motorway without problem but on the last 5 mins drive it lost the electrical power steering and all electrics were very week, when I got home I turned it off and tried to re start but the battery was totally flat, I put a charged battery on and everything was perfect again ! I used it for a few hours the next day until the power steering stopped working as did the radio and other electrical items but as I was close to home I got back an swapped to a fully charged battery and that solved all problems ?
Iv had the alternator and battery checked out an there ok so I don't know where to try next ? The only other thing I noticed is when I turn the engine off it says steering not locked but don't know if this is a seperate problem ? I'm desperate for help as Iv been off work for nearly 2 weeks due to someone driving into me an now it looks like I may have to take more time off

04 megane battery being drained in a few hours