Joists to binder – best way to attach

Q. Hard lines are the walls on the first floor underneath, joists on the first floor landing run left to right so does that mean the central wall running front to back is 'load bearing'?

If your house is in the UK (which I'm guessing it is), then without a doubt the central wall running from front to back is load-bearing.

Q. Initially I thought of putting 8x2s front to back, supporting them on internal walls where possible. Problem is that the angle of the eaves would not allow me to rest the ends on anything substantial so could I just bolt them to the rafters as long as they were supported by the walls downstairs too?

Refer to this question here on SE, and read all the answers and comments found within.

Running parallel to the existing ceiling joists is the easiest and preferred way to go.

Removing the binders "locally" to allow the new floor joists to pass is "normally" considered acceptable (then connecting/pinning them back to the new joists passing through), though you would want to check such a detail with your local Building Authority before doing so.

Otherwise if you go the other way (perpendicular), you end up going above the existing ceiling joists, which causes a major re-work of the existing wall plate area that sits on the exterior wall and the roof area above.

Q. Then I got thinking that I'd need to go left to right .. a) because the load bearing walls go down the centre of the house and b) because if I don't the ceiling binder will be in the way. Same problem however with regards to getting the joists to sit on the external walls and I'm guessing I'm not allowed to bolt a plate to the chimney wall and hang things off that.

Whatever you do, do not connect anything to the chimney wall. Doing so is an absolute no, no in the UK (if I have your location correct).

Again, refer to this question here on SE, and read all the answers and comments found within.

Of course another option is to remove the existing ceilings (lath and plaster), binders, etc. Leave the existing ceiling joists and install new appropriately sized floor/ceiling joists parallel to the existing, then fix a new plasterboard ceiling to the new joists. Granted, such an option adds considerable mess to the job, and a reasonable cost increase.

If you have space and there's any structure left near the ends, the easiest way is to not replace the existing rafters and joists, but to sister a second board on the side of each. Attaching the two with bolts every foot or two (50 cm). Use several bolts at the ends of the wider lumber (7x2).

If you don't have the space or the existing structure has rotted to the point that it won't hold a bolt, then a temporary support is very similar to the existing structure, built as close as possible to location you're working on. If there is a load bearing joint above your structure (unlikely for where you're working, this is typically seen with a load bearing wall and two joists meeting over top of the wall), you would build support for each side of the joint. If the existing structure has sagged, you'll want to jack it back up before installing your new support.

Note that when it comes to engineered joists and rafters, they are frequently designed with precisely engineered joints and webbing to carry the load. If you cannot match exactly what was done before, then I would recommend getting a professional, or at least the advice of an engineer.