I think I have sorted it out, so for any other UK keyboard users with this problem, here it is.
I was actually using a SVN-Daum build based on DOSBox 0.74, not native 0.74 itself, and I am playing Daggerfall.
I don't know why the CTRL-F1 DOSBox Keymapper has the US keyboard layout, even though it's set for UK layout in:
dosbox-SVN-Daum.conf in C:\Users\<UserName>\AppData\Local\DOSBox
and this is reflected by the commands etc that DOSBox loads in the DOS Window when launching.
Anyway, to remove the Keymapper function from F11, this is what I did:
press CTRL+F1
there is a box of functions and I clicked SPEEDLOCK2 and deleted it, so it's no longer associated with F11.
Now, I can get out of the Void in Daggerfall (YAY!)
For the key between L-SHFT and Z, which on a UK keyboard is \ but on the US layout <>, so I cannot map \ to JUMP in-game.
So I selected the \ key on the US layout (which I think is ~# on the US layout) and set it to be <> in the mapper, which is \ on my keyboard.
So now my \ key now outputs \, and I can map \ to JUMP in-game. Without doing this, when trying to map \ to JUMP in-game, the game thinks I am trying to map < and CTDs.
After doing this, a new file was created called mapper-SVN-Daum.map in
C:\Users\<UserName>\AppData\Local\DOSBox
Beforehand there was no such file and the only tools I have are to reset this mapper-SVN-Daum.map file, which deletes it. I cannot find what DOSBox is using for it's default setup with that pesky US layout, but the file is full of settings for keys, so I assume it is possible to create a UK setup by changing some of the numbers so that " £ | \ @ ' ~ # < > and so are are in the right place.
The mapper-SVN-Daum.map file contains loads of things like
key_k "key 107"
key_l "key 108"
key_backslash "key 60" "key 92"
and some of the Daggerfall functions are stored as well.
I am a total novice at this and maybe I may need a more recent version of the latest DOSBox which is out of beta and ready to roll. Preferably one where I can set the flipping keyboard to use UK by default!
So if anyone can advise, I'd be most grateful.
Cheers!