Central heating back boiler problem Cant refill system !

I have just moved in to a new house and hove problems with the central heating
radiators eiher didn't work or worked very badly
central heating is run off a backboiler from living room fire
i have taken all the radiators off and cleaned loads of thick black sludge out of them and all baring one is now running fine should be able to solve that easily enough
my problem is i don't know how to fill the system back up

i have done its mostly barring one radiator at the mminuteby connecting the garden hose up in place of a radiator and filling and bbleedingthe system from there then turning the valve off and connecting the radiator back in but this leaves me with a rradiatorswworthof air in ( or more!)
i cant find a filler loop anyware i hhaven'tdone any thing about pulling the fire out to look for it as i don't think it will be hidden back there
the system is quite old and run off 10mm copper pipe there is a small header tank in the loft which is full the water llooksclear but there llooksto be a lot of rust in the bbottomof the plastic tub could this be blocked from here in to the system dose it fill from here ?

also this small tubs overflow goes directly in a large tub which i assume feeds the rest of the house water i assume this to be bad as for potententil contamination if the rads feed back to this if its even anything to do with the ch system i haven't bean able to trace many of the pipes as the loft has bean converted hiding most the pipe work in the loft

i am at a loss with this and dont know where to go from here !

blocked cold feed where pipe from loft tank meets pipe work in A/Cupboard

After that you have another problem to solve; you have to clean all the pipes

System is likely to be filled from the small cistern (tub!) in your loft. There will be a pipe, likely 15mm from this, into the system somewhere, usually connects into the return pipe from hot water cylinder to boiler. This likely is blocked, will need cutting out and either replace or clean and refit affected section. Would be wise to clean out the cistern while your doing it, system should refill by itself then.

Sounds like whole system would benefit from a good flush, as said, pipes will be full of sludge as well as the boiler, if the rads are that bad. It is of no benefit to leave your system in that condition, ideally it needs a good flush, then refilling and inhibitor adding to prevent any further damage.

Also, i'd remove that overflow from your cold storage cistern and reroute it to outside. Really dont want water from central heating system in the other tank, at a minimum it supplies your hot water tank, thus hot taps. Do NOT move the pipes that loop over the tanks in an upside down 'U'.

Thanks for the suggestions
I thought this was most likely the case
now just got to try and find the blockage that should be fun !
as soon as i saw the tank over flowing in to house tank i knew it was wrong
an it will be changes as soon as possibale until then i aint drinking the water !