There's a small-bore hose on my car that runs from the top of the header tank and goes into the engine (VVC) between the inlet manifold and the dipstick. What's it for?

There's a small spigot top right rear of the cylinder head, it's to allow any air trapped in the top of the head to escape up into the the expansion tank. There is a jiggle valve in the spigot that facilitates this. It's a little plastic ball that floats up to block coolant from returning to the exp tank via the head*, but if there is air the ball falls down and allows the air to pass into the tank then when coolant replaces the air the ball floats up and blocks it.

* to stop the exp tank from being part of the coolant circuit.

I think the ball in mine is missing as this is squirting all the time back to the expansion tank. Other than being a short circuit I wasn't too concerned as I definitely won't getting airlocks in the head. Any thoughts on whether I should sort this?

It could just be gunge stopping the ball from floating.

IMO it's definitely not ideal as I think it will unduly pressurise the exp bottle and you might be losing some 'normal' flow to this circuit.

If your coolant temps are stable when driving and you're not racing etc you'll probably be ok, but I would get it looked at soon as you do other work on the cooling system.

If you want it looking at now on it's own i'd try clamping off all the hoses feeding the engine and you should only lose what's in the exp bottle and whatever is in the head - rather than the faff of (refilling/bleeding) draining the whole system.

Is your intake manifold metal or plastic? I believe the plassy ones have more problems with this.

If you just take the hose off (water water everywhere!) and poke a thin rod down it should be stopped by the ball. To actually get at it you have to take the intake manifold off. Think you might need to drain coolant out of the head at least first.

Mine is the ali manifold, since putting the 160 in it is the later mems3 plenum with the combined MAP and temp sensor. I actually had the manifold off just a few weeks ago to cure a vacuum leak and I replaced the manifold to head gasket. At the time it didn't strike me but having looked at some images of the manifolds the jiggle valve should be mounted in the manifold. I must have assumed at the time it was in the head. My manifold just had the hose connection and an open space. I have read on some MGF forums that it is not uncommon for garages to remove the jiggle valves when they do a HG replacement as many think that as they are very prone to sticking shut you are better off with the lesser evil of constant flow than a blocked jiggle valve. As such I don't think my jiggle valve is stuck open, I think it is missing alltogether!

It'll stay as it is for now. I can't find jiggle valves for sale on their own. I have a spare of the older ali inlet manifold (with the VVC lettering and separate MAP and temp sensors) so I might be able to either liberate the jiggle valve from it or just swap over the bottom section one day.

Would a restrictor be an alternative to the ball? This would pass large volumes of air or steam but not much liquid water.