How to locate blockage in CH system? - help please

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.
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wrote:

1)    Are the boiler and woodburner 'in series' or 'in parallel' to use electrical terminology? 2)    Do I understand that you have two circulating pumps?
    Presumably you can tell from the vibration whether the pump is working on the boiler circuit? (Also, presumably the ch outlet from the boiler is cold if the boiler is on hw only?)     If that pump is working, I think the heat sources have to be in parallel if the wood burner can heat all the radiators but not the boiler. If so, any blockage is in the piping which is NOT common to the two circuits.     Then, the most likely position sounds to be in the boiler inlet if the faulty valve (replaced) was close to the boiler in the ch return.     Or am I missing something?
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They may be a strainer on the return inlet to the boiler. If so, remove the cap and clear the gunge (and valve flap) from it.
Christian.
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