Thank you for all the pointers!
Sorry for the late reply, I was out of London so couldn't check the bike.
If anyone knows a good mechanic in London I'd be grateful for the recommendation.
I don't have a stand, but as far as I can tell, the chainring is not far off from the middle of the cassette. The mechanic said it's placed where the old small chainring was placed.
I went for a ride yesterday, and unfortunately in addition to the chain falling off at high gears, it also doesn't change to the two lowest gears while climbing.
The chain always falls outwards, towards the pedal.
Is it possible that both the gears not changing the chain falling off caused by a short chain? I have a 114 sram apex chain that is meant for a 44T chain ring, while the one I have is 46T.
I'll definitely try the chainring suggested, if anything else can't help.
Thanks again for all the help!