Loose ceiling joists

Initially you asked if your roof was safe, you then went on to refer that not all rafters and ceiling joists were in the same plane (am I right?) therefore the first thing to consider is binding rafter to joist forming a truss of sorts to each rafter, even though they are not integral. This the binders will achieve.
OK, first the nailing of the the joists stops any lateral movement, so if the joists are set at 18" for instance and if all the joists are nailed to that same batten they will stay at that setting for ever and a day. Same applies on the opposite side.
Now fixing the rafters to that same batten of wood counteracts any sliding stress preventing the rafters from splaying outwards. Though not a perfect scenario, when tied to the stone building with tie-down straps there should be no discernible movement.

[I,m going to try and establish some insights into your present roof configuration from what you posted initially and subsequent posts, using my experience both as a joiner who worked on many roofs, a roofer and a projects manager in construction of said ]
15 -20 years ago the roof was redone, we cannot know exactly what was either done nor what the original configuration was. All that can be ascertained is that this was a traditional rafter designed roof. Rafter design was borne from centuries of skill and knowledge and it was not uncommon to find these roofs siting directly on top of the wall heads (the weight which is considerable holds everything in place) Come the 1940's the government faced with all sorts of shortages, introduced new rules for roof construction. Out went the traditional rafter design, in came the lightweight truss design, which is prevalent today. These rely on the manufacturer to design/build a unit balancing all applied loads (both natural and applied) within each truss, are delivered to site and erected. Rafter built, were cut and built in situ.
I suspect that during the re roofing operation the rafters were either replaced or altered to place them on a wallplate (possibly birds mouthed over?) this would lend it's self to the wallplates being outside the length of the ceiling joists (close examination may even show a damp proof membrane under the wallplate!) If the rafters were set to a different setting to the ceiling joists, (say metric as apposed to imperial) this would give rise to ~ 'if only the ceiling joists coincided with the rafter tails' [/img]
and why the ceiling joists are not on the wallplate as should be expected.

I hope I'm being helpful here...pinenot :unsure:
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Interesting info there about the change to truss roofs - didn't know that!

I understood that the binders would keep the correct spacing between the joists, but would you able to detail the best method of fixing the new binder to the rafters - that's the bit I'm struggling with :confused:. For example, would a fairly standard angle bracket like this or this suffice? Showing our novice-ness here I suppose :oops:

Thanks for all your help so far :D
Initially I suggested nailing them, this would come up through the bottom most face of the binder into the touching arris of the rafter or there about's. You might find that difficult so you could screw these if preferred, make sure the binders are fully fixed to the top of the joists first. If using nails this will give a sound fixing, but I can understand that hammering a nail in upside down might not be everybody's cup of tea.
There's no need for any sort of metal brackets in this operation and quite honestly I would eliminate the hold down straps unless specified by an engineer.

Cheers...pinenot :)

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