I have a conventional gas fired hw/ch system. Old boiler but working
perfectly well. Tanks in loft, hw cyl in airing cupboard (renewed this
year). Pumped ch & gravity hw. Motorised valves on both circuits, cyl stat
& room stat. Zone valve to control upstairs heating. Thermovalves on all
rads. All wiring goes to a multiconnector board and thence to the
programmer. Thats the general layout which gives separate control of hw &
ch.
Earlier this year I found that when the gas was heating HW only, the rads
were warming. Heating engineer diagnosed the 1-way valve in the ch system
faulty. This was an old barrel-type valve in the ch return close to the
boiler.
Replaced valve. Rads no longer warming.
When the old 1-way valve was removed, the valve flap was missing and is
presumably trapped somewhere in the ch system.
Just to complicate the matter, I also have a woodburner plumbed into the
system. This is pumped, heats the hw and is also connected to the ch circuit
by a motorised valve. The system was working perfectly last winter.

Finally gets to the current problem.
On gas only, I'm getting little or no heat to the rads but the primary ch
flow is certainly hot near the boiler, the return is almost cold. I have
tried turning up the stat on the boiler but this makes no difference.
When the woodburner is alight and set to ch the rads are heating well. I am
presuming that the missing valve flap is causing a blockage in the ch flow.
If this thought is correct, how on earth can I find where the blockage is
located? Or could it be something else?

Any help or advice would be much appreciated.

Nic.
Apologies for rambling.