Hi,

That are correct or more than likely to be.

What is happening the water is holding in the spout under a vacuum then lets go and the water runs out.

The reason the hot doesn't do it, is because it comes from a separate tube, in the center of the cold.

Give the tap (spout) a little tap next time you turn them off.

I would suggest 40ml of water is a very small amount and probably typical for many kitchen mixer taps. I bet most people waste many times that running the cold tap before putting their glass under it.

According to one water co we each use an average of 160 litres of water each day at a cost of 55p. That means water costs about 0.3p per liter. So each 40ml wasted costs only 0.012p  I suppose if you used the tap 100 times a day that works out at about 1.2p a day or £4.40 a year.

You may find that as the tap scales up slightly the spout starts to hold some of the water rather than allowing it to drain out. But would you really want to drink water that's been sitting in the spout? It might have come from the hot feed depending on who used which tap last.

It's possible to worry too much about water saving

Thank you for your advice re the amount of water I'm lossing - it's not a lot when you do the calculations.

The main thing I wanted to check was if the pulmber was correct in what he was saying about the tap drippping even though this doesn't happen with the hot tap.  Do you have thoughts on this?  I just want to feel like they aren't trying to take advantage because I know nothing technical about plumbing!

Thanks for your time, best regards