Trickling sound in BMII

First of all Mike, I propose you for an award for services to householders! At last I have found somewhere that talks about BMIIs in a manner (I think) I understand.

A) The BMII in my (let) house has started to make a trickling noise whenever the central heating/boiler is running (unfortunately I did not establish when visiting from some distance yesterday) whether it correlated with when the boiler was firing or with the radiator circuit pump running. The ball valve in header seemed to be working and there was water in the header tank. There appeared to be a magnetic scale inhibitor in the cold supply but some scaling or rusting texture on the ball valve arm (system is about 10 years old). After your excellent notes on BMII I think it must have been a trickle from water returning to the tank (not coil) implying a block in the small L pipe (see A-edit below) from header tank to main tank being blocked. Although you think it will be difficult, I should presumably first try to get a pipe cleaner or not too stiff wire into and round the little pipe from the access in the top of the header tank. Do you agree? Could the trickling noise be anything else?
A-Edit: Further research points to the top-up pipe from header to main tank reaching all the way down to the bottom of the main tank, so possibly blockage is accumulated sludge in main tank. Has anyone ever tried feeding a 10-5mm o/d polythene tube down inside the copper and then blowing hard to blow the scale away from that part of the bottom of the tank (wishful thinking?)?

B) Quite separately, while inspecting the system, which delivers hot water to taps and radiators perfectly efficiently, I noticed quite considerable warming back along the mains cold supply pipe reaching back (maybe 30-40cm) from the point at which it linked to the blender valve. We were not running taps in our test, so I suppose it might just have been conduction along the copper or water, but it was over 60degC 15cm back from the valve body. Might that be associated with a malfunction in the blender valve?

C) If I do unblock the header to main tank link and that solves the trickling sound, should I insist on dosing the tank/boiler circuit with an inhibitor chemical?

D) Can I ecommend that householders may find it useful to learn about BMII before taking up plumbers' time by reading user and installation manuals to understand principle of BMII.
http://www.gledhill.net/pdfmanuals/Older%20Ranges/BoilerMateII-Scanned.pdf
http://www.gledhill.net/pdfmanuals/User%20Instructions/BoilerMate%20II%20User.pdf

Thanks in anticipation of yr reply.

Hi purselove,

You have the dreaded blocked internal cold fill pipe. The store water is gradually leaking away and not being replaced, hence the waterfall sounds.

The surefire fix is to install a new, external fill pipe from the header tank down to the drain point at the bottom. Occasionally you can clear the blockage in the internal pipe by connecting a mains pressure hose pipe to it but only very rarely.

Yes the hot feed pipe is just conduction. Ignore it!

Cheers, Mike

Thanks, Mike for clear and authoritative reply. I'l try hose and then if needed get a professional to fit the external filler. Would I be taking a very big risk if I do not add inhibitor when the top up is working again? I know you can't recommend that I don't do it, but would it be really daft not to do it? If so, do I just add a chemical, if so which?

I'm afraid my past experience was on nothing smaller than 30MW and even that's pretty rusty now (if not completely scaled over)!

Should I be recommendeing you for an OBE or a knighthood?

Had similar problems to Purslove. Changed boiler circulating pump. On start up had no heating unable to bleed. After reading this forum I guessed I had a blocked cold feed pipe. Removed F&E tank and found the pipe blocked. Using a thin piece of wire managed to poke through the sludge. I then poured boiling water down the pipe and left for an hour, on return found the pipe clear. Reassembled F&E tank, filled up the tank. Radiators bled fine all now works.