Toilet cistern water keeps flowing, flusher not working

he grey clip and the running water are almost certainly linked — here's what has happened and what to do:
About the grey clip
Your McDee (Macdee) system is a siphon flush mechanism. The small grey clip is most likely one of these:
A retaining clip that holds the push-button rods or linkage in place — it sits on the vertical stem and keeps the rods aligned so they push down squarely
A limit/stop clip on the fill valve stem that sets the correct water level height
A connection clip that joins the button rod to the siphon trigger mechanism
Without it, the button presses but nothing lifts properly — hence the button feels dead, and the mechanism has stuck open letting water run through.
How to fix it
Turn off the water supply — isolation valve beside the cistern
Empty the tank — flush and sponge out remaining water
Locate where the clip belongs — look for:
A thin vertical rod coming down from the button — the clip should be on this rod to keep it centred
The siphon trigger at the bottom — the clip connects the rod to the trigger arm
The fill valve float stem — a clip here sets the water height
If you cannot find where it fits — the good news is Macdee siphons are a standard, widely available part. A replacement Metro siphon (the common McDee model) costs about £20–£30 and comes complete with all new clips, rods, and seals. Fitting a new one takes 20 minutes and gives you everything fresh.
To stop the water running now
The water is flowing because the flush seal at the bottom is not seating properly — likely because the button mechanism is stuck down or misaligned without the clip
If you can't refit the clip, you can temporarily stop the flow by gently pushing the internal mechanism fully closed with your finger, then turning the water supply back on slowly — but this is not a permanent fix
The reliable solution is to fit a replacement Macdee Metro siphon — it's a standard part available from any plumbers' merchant, just tell them it's a Macdee Metro push-button siphon and they will supply the right one

If the push button no longer works and the cistern keeps running, the flush mechanism is likely stuck or the grey clip has come off the button/linkage assembly. Turn off the water, remove the cistern lid, and check whether the button rod or flush valve has disconnected or broken. On very old McDee systems, replacement parts may be hard to find, so replacing the flush valve or the entire cistern may be the most practical fix.

This clip would have come off of the flush valve or fill valve system

Possible Cause:
The flushing mechanism (syphon or valve) has either failed or become disconnected, especially if the small grey clip came loose.
That grey clip probably held part of the flushing assembly in place, possibly the push button linkage or part of the flush valve.
The continuous water flow means the flush valve is stuck open or the seal is damaged or not closing properly.
? What You Can Try:
Turn off the water supply to the cistern (usually a valve near the base of the toilet).
Take off the cistern lid and inspect the flush mechanism.
Look for a disconnected rod, button arm, or broken part where the clip might have attached.
If you can temporarily reposition the flush valve manually, you may stop the flow until it’s repaired or replaced.
???? Repair Options:
If the clip can't be reattached or part is broken, you may need to replace the flush valve or syphon.
McDee parts are less common now, but universal push button flush valves can usually be fitted as replacements (Fluidmaster and Siamp are good options).
It might be a good time to consider upgrading the internal cistern components if the system is very old.

It sounds like the flush mechanism inside your old McDee toilet has either broken or become dislodged. The constant flow of water usually indicates that the flush valve (or syphon) is not sealing properly after the flush. This is often caused by a worn or misaligned seal, or a broken internal component.

The small grey clip you found likely came from the flush valve or the button mechanism. In many older McDee systems, these clips help secure parts of the actuator or syphon in place. If this clip has come off, it could be the reason the button is no longer working and the valve isn’t closing properly.