Air in central heating - where from?

Quite a bit of fresh orange rust up there. If you feel like it, tie up the float and bale it out (use a sponge to get the last bit out). How sure are you that the water still contains inhibitor? Draw a bit of the system water out from a drain-off into a jam jar, put a steel nail in and see if it rusts.

Also can you "Try tying a jar under the vent pipe (the one bent over the F&E tank) so that, if any water comes out of it, the jar will be filled. Water should never come out of the Vent pipe unless something is wrong. You can also sit up there with a torch watching the F&E, with the CH pump running at full speed and the boiler turned up hot. This is when interesting things are most likely to happen."

And have a look at the pump body, there will be an arrow on it to indicate direction of flow. Does it point up or down?

Thanks again guys.
WIll have a look tonight re the direction of the pump.
Will also take another pic of the tank cold - the heating was on when I took it.

As for inhibitor, the rads were removed and flushed about a year ago. It was cleansed for 2 weeks, then emptied and refilled and inhibitor added.

About 3 weeks ago it was all drained down and I put new inhibitor in then. I must admit I'd not really noticed the rust colour. I did give the tank a bit of a clean at the time it was first emptied so am suprised it is there already. Maybe I just cant see it without the camera flash.

Recently when the system was cleaned the water was a bit off colour but more clear than black as it was the first time.

Oh, and I tried the match trick when bleeding (despite not knowing if you were winding me up ) and nothing happened.

Thanks again for your help - I'll check what you have suggested.

If that pump is feeding down then the vent & feed are within the correct configaration. If the vent has only been extended recently u may have solved the problem but not given enough time 4 the system to settle down. with the vent lower it is common 4 the vent to run over each time the pump shuts down causing airiation to the system. if the system was drained all that fresh water will contain lots of oxygen that needs to be displaced while running

Thanks
I can confirm it is pumping downwards.
I'm also pretty certain water is not coming out of the vent pipe as I have been up there a bit over the weekend, though I never got as far as putting a jamjar under it.

If it doesn't turn out to be air in the water from the recent water fill, what is the next thing to try? Am I back to changing the pump? Any other things to try.

Rather than further 'shooting in the dark', give Fernox (Cookson Electronics) or Aqueous Logic a call and arrange for a test of the water in your system. There might be bacterial activity and /or electrolytic action causing gassing.

On the face of it, there is PROBABLY nothing wrong with you plumbing as such. But if you didn't change it as part of the current investigations, why is the vent pipe over the F&E routed up so high??? Looks like there's something going on that's been a problem for a while.

Where is the highest radiator with respect to the F&E tank?