A plastic pipe in the eaves leaking with a constant flow is a sign of problems with the hot water storage tanks.
If the water level in a tank is too high the excess water will constantly flow from the overflow pipe. Go into the loft and take the lid off each tank. There are normally two but can be more depending on the size and type of system you have. Take the lid off each tank and check how high the water level is in each one.
The water level should be several cm below where the brass part of the ball valve reaches.
Ball valve passing
This is by far the most likely cause. If the ball valve is passing there will be a constant drip from the valve into the storage tank in the loft. You will need to change the washer or replace the whole valve. I always find that changing the whole valve is more reliable than just a washer.
Ball float split
If the ball float on the end of the valve has split and filled with water the float won’t float so the valve will not fully shut off. The ball float unscrews from the arm of the valve (anticlockwise) and a new one should screw on.
If the float is heavy and sounds like it’s full of water its split. It’s a standard thread so any specific ball float should fit.
Without a ball float on the arm of the valve the tank will fill continuously. Either turn off the water at the stop tap or tie up the arm so it can’t drop.
Coil in hot water cylinder split
If the coil in the hot water cylinder has split the water of the heating system and hot water cylinder will mix and try to find their own level.
Water from the mains or from the highest header tank flows into the pipework and overfills the feed tank.
This will make the lowest of the tanks overflow.
The only reasonable solution to this is to replace the hot water cylinder. This is a big and expensive job so take a lot of care confirming this is actually the cause.
Passing thermostatic mixing valves
The final reason the plastic pipe is leaking from eaves with constant flow is a faulty TMV. Thermostatic mixing valves (TMV) are used to avoid scalding water being delivered to hot taps.
They work by mixing some hot water with some cold water until the desired temperature is reached.
Both the hot and cold supplies to the valve have none return valves which are supposed to stop reverse flow.
The non return valves can stick open which allows the higher pressure water (normally the cold from the mains) to travel up the hot pipe to the hot water tank.
This can be diagnosed by feeling the hot pipe feeding the tap when the tap is running. If it gets colder in comparison to other hot water pipes the cold is flowing up it.