To make matters more complicated there is water UFH in each room (just more work) that I need to navigate around.
I think you may have a hard choice - if you want structural stiffness and flatness you will probably need to lose the UFH if it gets in the way (odds are it does)
On the other hand there are suspended ceilings on many of the floors making it easy to deal with each joist individually without destroying the ceiling below.
Is the original lath snd plaster still attached to the joists? If so the weight of a section of that dropping onto a suspended ceiling on MF could cause a collapse. In addition that MF will more than likely have support hangers dropping from the existing joists (or a track attached to the underside of the joists to which the hangers are attached), so if you try moving the original joists upwards in any way you'll possibly wreck the ceiling. These hangers hold the primary profiles in place every 1200mm and the top hat sections which actually carry the plasterboard are clipped or screwed to the underside of them on 600mm centres (mostly)
Is it possible / easy to lift up each joist and pad them at the ends to level the floor. Do these joists usually only span one room, so I don't need to worry about cutting them at the spine wall? How are they fixed at the spine wall end? I assume at the brick end they are bedded into the wall?
See above...
The joists might go through the whole way - I've come across 30 foot long joists before (albeit rarely) - they might be supported by the stud wall (which, incidentally may need strengthening). Almost certainly the ends by the outside walls will be pocketed into the masonry - a notorious point for wet rot. If there are signs of wet rot new timbers running to such a wall need to be pressure or vac-vac treated
Alternatively I could bolt a thinner piece of new timber (say 1 inch) to the current joists . I guess in a few I could also add a fitch plates to increase rigidity as well as noggins between joists.
Sorry, but adding an inch of timber, either to the top or the side won't do a fat lot of good. Neither will plywood
Steel flitch plates bring with them the need to calcate the loading, additional bow in the centre of the span, etc. I'm not an S/E, but I have never seen them used on domestic scale structures other than in former (large) shops with living accommodation above and in structures sych as banks, hotels, warehouses, etc where there are "double floors" (i.e. large section primary timber beams running across the buildings with joists suspended between adjacent prinary beams). I suspect the additional weight would actually ADD go joist bowing in the average Victorian or Edwardian terraced house. BTW, if you'd ever manhandled steel flitch plates into place, even with a Genie lift, you probably would not have suggested it...
By noggins I presume you mean solid strutting. Noggins have no defined size relationship with the joists - dolid strutting should be the same cross section as the existing joists if at all possible, but in sny case they should be at least the same thickness and no.less than 80% of the depth of the original joists (so for 7 x 2in joists than means at least 5.6 x 2in). Noggins much reduce the tendency of joists to twist under load and in so doing they stiffen the floor.
TBH your best bet will be to sister the joists with new joists of the same cross section, the new joists being bolted to the old and pocketed into the masonry where they meet masonry walls (and enveloped wrapped with an impervious material where they go into any masonry to minimise rot). C16 is the lowest structural grading you can normally get, but if your joists are undersize (based on NHBC joist tables) you may need to install C24 iinstead.of course, to install these the skirtings willneed to come off and the floorboards will need to be lifted first
Is there a better solution that I am not thinking about?
One, maybe. Clad the floors with 12mm plywood, screwed at 150mm centres (that's 150 odd screws a sheet), then level the floor with self levelling compound. Quick and dirty, won't make the floor stiffer, but will get you nearer to level providing your existing floor isn't really horrendous. Your UF CH might even still work with this one
There is one other alternative which I have used on commercial buildings - the floors are ovwr-framed with 2 x 2 and 3 x 2 CLS "frames (basically stud walls laid flat on the floor and packed up level (on packing shims) before being fixed in position with angle plates and screwfs, then overboarded with T&G chipboard. The downsides tonthis are that itvreduves head height in the rooms (so OK in a 10ft high room, but not in a lower one), the door casings/linings will need to be lifted to the new floor height (as will the skirtings), it increases the load on the floor (so may worsen bowing), and you can kiss goodbye to your UF CH working
I've done far too much of this sort of work over the years to recommend it to DIYers, other than the keenest. From the foregoing you can hopefully make your own mind up as to howcfar you want to go. Other than that, I'm with -
Personally I think you are opening up a massive can of worms trying to level joists in a Victorian House, unless the house is empty and you are willing to remove all of the skirting boards all of the flooring and replace all of the ceilings...
...The ground floor is slightly easier, but still a huge under taking.
