Cold water conundrum

[Stones]

Around the turn of the year  we experienced what I would describe as a brief moment of water hammer when we turned on our bathroom basin cold water tap, followed by much lower flow of cold water than normal.  All other cold taps in the house were at normal pressure.  I checked the aerator and swapped out the cartridge but this made no difference.  Working back from the tap, I tried turning the isolator on / off several times, but again no difference.  One thing I noticed was that our toilet cistern was filling much slower than normal during this period, indicating the constriction to the water supply was somewhere before the branch that feeds that toilet and sink.  After a couple of weeks the pressure went back to normal and everything seemed fine.

 

We returned from holiday yesterday (during which time the water had been turned off at the stopcock).  On running the shower, there was a brief moment of water hammer before the cold flow reduced.

 

As with the basin before, hot water unaffected.

 

I can only surmise that there is some sort of constriction in the cold water branch pipe that feeds this particular bathroom (end of the pipe run).  

 

All other cold outlets in the house remain full pressure. 

 

The question is what can I do to try and resolve the issue, assuming there is indeed some sort of constriction in the copper pipe?

[Gone West]

The only similar thing that happened to me was a small plastic cap in the double check valve detached itself, and intermittently blocked the flow through the water softener. I just disconnected the larger pipe work and flushed it through. Is there a reducer of some kind before the cistern and tap that could be removed.