I am wanting to insulate and board out an area of my loft. The roof is a cut roof construction. The ceiling joists are 4 x 2. I need to increase the thickness to 8 x 2 to get the insulation I want in and to have a flat floor that will clear all the pipework in the area. The span of the joists is 2.1m. Do I need to structurally join any timber to the existing joists or should these be of sufficient strenth to merely screw or join them together?

I will be using 18mm chipboard loft boarding and intend on storing miscellaneous loft junk on the area.

You can screw the 2x8 joist along the 2x4's. Just as long as you place the ends of the new joists on top of the walls. If you don't support the ends and just screw along side of the 2x4's they will sag due to the weight of the stored items. (depending how heavy those items will be).