I'm sure you'd be able to find a tank to fit through the gap....it just might take a little longer to find the right one.
Then when you do water changes get some extra long syphon tubing and put the bucket at the bottom of the steps. Then Mum can suck to get it going while you're in the loft with the other end
Have a look around at the clearseal tanks. They make some of those quite narrow and they're much cheaper than normal tanks because they don't have all the built in lighting and stuff.
I hope you manage to get things sorted and feel free shout if you need any help