I have a cheap Tanglewood Rebel 4K but the neck appears to be bent.
When I press down on the string on the 20th fret there is a sizeable gap between the strings and the frets in the middle of the neck.
Is this something that can be sorted out?
I was just going to sell it on but I do not want to sell someone something that is not playable.
My wife has shown an interest in also learning bass and the tanglewood is small bodied and light and wpold be perfect for my wife to learn on.
Paul
There are people in here who know WAY more about this than I do… But, to get you started:
the way I understand it, the neck HAS to be bent to a certain degree
of course, excessive bending is not good. Have you checked the truss rod in the neck and its tension?
a high action (the gap you refer to) at the 20th fret might be adjusted (= made lower) in different ways. It depends a bit on how the action is closer to the nut/headstock, perhaps around the 4th fret. When doing adjustments, you need to monitor the action on one of the lower frets (i.e., 4th) as well as around the 12th fret, after each tiny adjustment step.
There are videos on YouTube on how to set up a bass, but if you don’t feel comfortable about this (and I totally understand this) then maybe find a local store or luthier to do this for you. On the other hand, if it is indeed a cheap bass, and perhaps it is already almost “beyond repair”, you might gain some valuable experience experimenting with it/on it.
I’d recommend taking it to a guitar shop that sells basses and asking them if they can set it up for you. To me it sounds like it probably just needs to have its truss rod adjusted a bit. If you fret it at the first fret and the fret where it joins the neck and there is still a large gap at around the 7th fret, then this is very likely the case. The gap should be about the thickness of a credit card or so. This is usually an easy adjustment.
You can do it yourself as well (it’s not hard at all); you will want to learn to eventually. However for a first time it’s probably good to have someone experienced do it.
I like this video a lot, it explains the whole setup process well. The truss rod part starts at about 7:30.
agree, as always 
@studio - Like others have mentioned, all you might need is a simple truss rod adjustment. Once you do that, then adjust the action of the strings. I’ve done a bunch of them on both my guitars and bass’s - pretty straight forward and once you do it a couple times you’ll get a better feel of your instrument.
Good Luck!
Keep on Thumpin’!
Lanny