Unusual plumbing: Does anyone know what this is?

It's on my Mother in Law's house, and has started spraying water at high pressure out of it, about once every three days. She has discovered that hitting it with a can opener stops it, which is not especially a problem, unless she's out...
She doesn't know if it's hot water or cold water coming out, or if it's on the HW or CH system.

For some reason, the pictures below got rotated when I uploaded them, so apologies if they appear at an odd angle. I have labeled them to help.

'Picture1' shows the device in question - the silver cylinder with the red blob on top. 'Picture2' shows a wider picture - there's a pump to the left of the devce, and a zone valve to the left of that.

The cupboard it's in used to house the boiler, until it was replaced by a condensing boiler which is now in the garage. The house was built in 1969, and there seems to be no effect on hot water supply or CH. A pro plumber has had a look, but he's never seen anything like it before, and wants to know what it is before he removes it.

My feeling is to just remove it. If any problems are caused by that, then we can fix them, but at least there will be no fountains in the hallway in the short term.

Does anyone know what it is?

It's an automatic air vent....

http://www.intatec.co.uk/products/automatic-air-separators

I'm not sure if you can unscrew the red dust cap and temporarily screw down the vent.

Looking at the pipework it will help to remove air since you appear to have a combined feed and vent system (that by their nature do not vent air so well)

Brilliant! Thanks mate!

Is there a problem with the vent, then, or could the spraying water be a symptom of a deeper problem?

The valve seating inside is contaminated with debris...all air vents suffer this way and eventually leak...replacement is probably the only option.

That's great - thanks so much.