Hello napier
Unibond super PVA might be suitable. This adhesive would allow repositioning of pieces to ensure they are in the correct place, but would require holding in place with tape, pins. or cramps (small modelling types) for about 30 mins if there is not too much pressure on the joint. The adhesive is applied to one face only. One of the faces must be porous.A contact adhesive where the glue is applied to both faces and allowed to dry first and then the pieces are put together would provide a more instant fix but this would mean the pieces would have to be offered up to each other in the correct position. The PVA glue is easy to apply and comes in tubs and tubes. Before it is set any excess can be wiped away with a damp cloth.Some super glues might also do the job for you when pieces are small and can be held in position by hand for a few seconds but not where the pieces are large and require lots of glue to be spread. Hope this provides a starting point for you to do your research as to which type you think might be most suitable. Just thought, if it is model to use on water then a waterproof adhesive like PVA would have to be used!!!!Good luck with the model.