Tepid water after new kitchen installation

[PhilH]

After installation of new kitchen, hot water no longer as hot as it used to be. The only plumbing changes during kitchen installation, were new main stopcock and extension of HW and CW pipes (by about 15 inches) to new mixer tap for sink.
After investigation of boiler (Vaillant combi condenser 828 - 20 months old)) it transpires the HW coming out of the boiler is hotter than that coming out of taps. Therefore the boiler is functioning correctly (confirmed by Vaillant engineer). Shutting off cold water inlet to boiler should stop flow from HW taps but cold water continues to flow (at a slower rate). Opening of a CW tap when this is happening reduces the flow, suggesting that the CW and HW systems are connected when they shouldn't be. I have isolated the sink mixer tap and the washing machine in the kitchen to no avail. The only other mixer tap in the house is the shower (thermostatic mixer) which I can't isolate. However it would be an outrageous coincidence that the shower mixer is the problem.
I have tried turning the mains water pressure down to a minimum. This helps slightly but doesn't solve the problem of cold water coming through the hot water taps when the boiler is isolated. Any advice greatly appreciated as Vaillant engineer and plumber stumped.

 

[Agile]

Its quite simple and you have correctly worked out whats wrong.

Cross coupling is usually a faulty mixer tap or shower or a wrongly connected "Y" adapter on a washing machine.

You can sometimes hear the water flowing through a shower mixer with a listening stick. Sometimes moving the temperature adjustment alters the eronious flow.

Tony