This depends on the actual NiMH cells you are using, to some extent..
My two-penneth:
Do NiMH need to be discharger before charging.
No, they do not suffer from 'memory effect' like NiCd's..
or is it just preferable to discharge.
Discharge / charge cycling will maximise the life of the cells, but needs to be done properly.. Not discharged too far, and not left 'cooking' when charging..
Would a small discharge device i.e. battery holder and lamp be suitable.
Preferbaly, you need something a little more 'intelligent'. - Though you could discharge this way for a set time, or to a set potential acroos the cell(s)..
After discharge how long can they be left discharge before having a detrimental effect.
I would always store cells with a charge. - I have only had problems with cells stored discharged, or neglected for some considerable time. - Though NOT NiMH Low Self-Discharge cells, which I tend to use these days..
If unused how long will the hold there charge,
'How long is a piece of string'? - This depends if they are completely isolated, or there is a small discharge path.. LSD batteries should have 80% capacity after a year..
and will the need a trickle charge or complete cycle.
Personally, if less than around six months, I just use them.. If I know they will be getting a whole day's use, or have been left longer than that, then I would tend to cycle them with an intelligent charger..
The above represents how I treat LSD NiMH batteries..
Works for me, others may tell you differently..