New Mixer Shower

Hello - in need of help !

I'm fitting a thermostatic mixer shower for a friend, replacing an existing one.
Heating system is gravity fed, hot water cylinder on 1st floor, as is the new shower. Large cold water tank in the loft, which feeds the new shower. There are 2 other bathrooms/en suites, one of which also has a shower, but since the loft is partly boarded I'm unable to know for sure whether the other shower is also fed from the loft cold water tank.
To remove the old shower I turned off the water at the mains, and the hot water at the hw cylinder isolator. I then discovered the tank in the loft, when the flow from the exposed cold pipe didn't abate ! I turned off the isolator (large one with red wheel) and the flow reduced to a trickle, but didn't stop. I left it running away for at least an hour, but when it stil didn't stop I carried on anyway.
Fitted the new shower OK, but when I tried it out I had horrible low flow through the shower head. I was only getting hot water through (flow decreases to almost nothing when I turn the temperature down), so I suspect I've got an airlock in the cold water system.
Am I talking rubbish ?
If it is an airlock, how do I clear it?
What else do you guys need to know to help fix this?
I can't get any pictures until Tuesday, and I'd really like to have a few clues before then.
Many thanks

did the gate valve open as they are prone to snapping the spindle when closing / opening.

Seco - what tells me that it's opened? I didn't feel/hear any snap/crack (or pop...ho ho) and didn't feel anything other than the usual (?) resistance.
Apologies, I didn't mention that there is a pump on a cold water pipe, situated next to the hwc.

i take the hot is on the pump as well.
did you feel the gate valve come to a stop when you opened it or just keep turning?
where does the pipework go from the pump up into the loft or up through the wall to mixer etc ?

No - I can't see any hot water feed into the pump. Pump is inline, very similar to the ch pump. (you can probably tell from my inane ramblings I don't know what this pump does!)
Gate valve stops when I turn on and off.
Pipe in which pump is sited(which is in the airing cupboard) goes up into the loft (and I would guess to the cold water tank??).
I can see a feed from the cold water tank to the shower, and on that pipe is a gate valve.

Sorry to be an idiot - many thanks for your help.

if the pump is on a pipe to the tank, what bottom of tank or top ?
wheres the pipe from ?
so the pipe comes out the bottom of the tank drops down into shower valve.
is their an horizontal parts of the pipe that can get airlocked ?

pump is on a vertical pipe in the airing cupboard, next to the hwc. Pump at a height that is about 1/5th of the height of the cylinder.
There is a horizontal run of about 3-4 metres between the cold water tank and the vertical drop to the shower valve.
You think that might be airlocked? How would I clear it?

useally on a run like that i would put a bleed valve.

is there any compression fittings you can slacken off ?
turn the shower on cold and a garden hose down the inlet in the tank use mains pressure will force it out.
or i've used the mains water pipe that feeds the ball valve before and connected a hose to that.


can you put any pics of this pump on here.

Gotcha (I think). So I put a hose (attached to mains) physically into the pipe, by feeding it into the cold water tank and into the pipe?

yes turn the shower on cold and turn the mains on don't need to be full pelt even if the hose don't go up the pipe it will be enough to get the air out.

Seco - you're a star. Thanks for your help, I'll let you know how it goes.

Thanks again.