Mike

The floor may be level but more importantly is there any deflection (bounce). If there is then that could cause a problem. Firstly before putting anything down, make sure the floor is screwed down at 300mm centres and that the screws you use wont rust.
When you say the gaps aren't big how big do you mean as DITRA will only bridge a gap of three mm.
If you do use DITRA then use a rapidset adhesive to stick it to the floor and the same to stick the tiles down.
If the floor has no bounce the gaps are small and the floor is unlikely to move then probably the best thing to use would be a 2 part rubberised adhesive such as BAL fastflex of Weber WF21 (plenty more are available).