The motor is loosing water from the centre welsh plug behind the exhaust manifold. What size is the replacement plug and where can it be purchased

SOURCE: Hose from water pump to intake manifold above and behind water pu

this is a water pump bypass hose connector to the manifold, take the broken piece to a parts store and show it to them, it should have the threads still broken off in the manifold so explain that. To remove the broken piece ask for a EZ OUT to fit the piece you took to the store.
It is common for these to become brittle and break off but it is a simple fix.

SOURCE: how do you change a freeze plug in chevy 1500

I would reccomend removing the exhaust manifold, then disconnect both motor mounts and raise the engine as much as possible..you will appreiciate all the room you can get

SOURCE: 1993 Pajero V6 3 litre has a leaking welsh plug

Engine has to come out and I advised to ger all frost plugs replaced at the same time.
Cheers

SOURCE: 96' toyota corolla Exhaust Manifold Keeps Cracking

Sounds weird that this problem keep occurring over and over. Does it crack in the same spot every time?

Only two things I can think of that would cause this problem.
1. Extensive over heating, I've seen a exhaust manifold get glowing red hot because of a bad oxygen sensor.

2. Excessive Vibration. The whole exhaust line to the back of the car is moving around too much or too stiff. The exhaust line needs to move around enough to take the movement/vibration of the engine. I would check out for bad motor mounts and worn out exhaust pipe mounts.

SOURCE: replace exhaust manifold on 2002 civic LX is it hard to do?

no you can remove it without removing the rad. the gasket is metal reuse it.forgot u must temove the heatshield