Tap won't turn off

[Tapdancer]

Help! Last night I ran a bath and the hot water tap wouldn't turn off. As a result, I've had to turn all the water off and empty the tank. So now I have no water. The tap is a mixer with separate hot and cold knobs, which turn 90 degrees with on at one end and off at the other end. The taps are only 18 months old and because it happened so suddenly, I am not sure that it is a washer problem. Also, I can't undo the brass nut to get to the washer bit. Can anyone tell me: how I can ease the nut to get to the inside of the tap; and whether it is likely to be the washer or the seating. Can I do this job without having to go under the bath because that end of the bath is in a little alcove and is really hard to get in under to look. Please help because I'm really struggling with no water and I am skint so can't afford a plumber until pay day.

[ChrisR]

Hopefully you "Just" need a bigger spanner, then you'll find the tap valve has come apart and will go back together again.
More likely though is that it won't, and if it's not a common make you'll not get a new valve part for it