I seem to have a problem (crashing) now that my dms_gb_os25_2012.qc3 file is nearly 4Gb (I understand this is maximum Android file size) and am heading that way with the os50 file.

I only have about 800 sq km of os25 activated but have panned round the UK and this has created a pixelated map of most of England & Wales, I presume that has created this file size.

Waht can I do to reduce the size of this file?

If I delete it will the areas I have activated still remain if I download again?

I am running it on a Samsung Galaxy y Duos GT-S6102

Yes, if you delete the map and reload, the areas already activated will load without need for additional activation.