Hi Sun.
Well done for contacting your Housing Assoc. Was this done in writing? If not, I'd suggest doing so again, so you have a record of having informed them. I presume both you and the person above are tenants? If so, you'll undoubtedly want that person removed after this is sorted, and lack of action from the HA at this point can hopefully be used to oblige them to make this happen.
To begin with, tho', I'd suggest you actually want to nail this guy properly, and have them sorted out, convicted, and removed, and not just for them to be scared into stopping, getting away with it, and perhaps even restarting later on? Good. Then you need 'evidence' - see below - and not for anyone to act until you have this. So hold off any other action for the moment, until you have evidence.
Obviously, like other users of this forum, I do not know the 'facts' here, only what you are telling us. But, I think answers should be given on the basis that what we are being told is correct and 'true' to the best of an OP's knowledge, unless there is very good reason to suspect the opposite. It is ultimately self-regulating in any case, as any helpful information given will be useless to anyone who is wrong about the original issue! So, I'm assuming you are giving an accurate account. I'm also not going to go 'conspiracy theorist' and pondering whether the HA is also in on this - that is highly unlikely.
Where is your meter and CU located - is it inside your flat? If it's easily accessible, then I think there may be a simple way of determining whether your power is being pinched. You will need a clamp-on energy meter, which you should fit over the incoming cable, probably between the meter and your CU. No idea which models are available these days, but I used to have a 'Wattson' before I sold it for a £enner, so similar must surely be available. Clamp this on, and it'll provide an instant readout of your wattage use. The Wattson was wireless - as most are - so you can keep an eye on your flat's totsal consumption in real time at a glance. Turn everything off in your flat, and confirm it's reading '0'. Turn on items of known consumption - say a couple of light bulbs, then a 1-bar electric heater (400W or 1kW or whatever size it is), your TV, etc. What energy is actually being consumed by each? Write down your results - better still, also video record yourself on your phone doing this with a narration; "Everything is off in my flat. Meter is reading zero. Turning on a single 1kW electric heater. Meter instantly shows 1,000W being used. Heater off. TV on - that's showing 120W. Just heard a click from above, and meter has jumped to 2,500W..." that sort of thing.
That is 'evidence'. With that, you contact your energy provider and tell them, "My meter appears to be faulty as I can prove that it is reading far higher - many times more - than any item I am using. Possibly there is another cause..." Ultimately, if the neighbour is doing this, they will be caught. I'd suggest you do want to 'catch' them, and not for them to just stop or ease off for a while and play it canny, knowing they can restart at any time. So, get a meter, set it up, live in your flat, and keep using power as you would normally - so that they, too, go back to normal stealing, if that is what's happening - and build up your evidence log.
Eg, if you are watching TV in the evening, note down everything being used - lights, TV, fridge, etc - and the combined wattage of these (that should be fairly easy to do). Your meter should read very close to this total. If the meter jumps, start recording - "Watching TV, meter was reading 235W until a moment ago when I heard a click from above, and reading has jumped to..."
