This is what I used on my Dusseldorf they are refrigurator water line Quick disconnects 1/4" 3/8" 5/16" Plastic Hose Quick Connector, Quick Shut-Off Hose Joint Tube Connector, Valved In-Line Coupling Insert Quick Connect Fittings (1 Male+ 1 Female) (1/4") Amazon

https://www.amazon.com/BBTUS-Connector-Shut-Off-Coupling-Fittings/dp/B0B1HMSSTF/ref=sr_1_12_sspa?hvadid=651150948385&hvdev=c&hvlocphy=9032905&hvnetw=g&hvqmt=e&hvrand=4651852174646533180&hvtargid=kwd-24170478216&hydadcr=23667_13619984&keywords=quick%2Bdisconnect%2Bwater%2Bhose%2Bfittings&qid=1680182514&sr=8-12-spons&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyQjQwNjRIQzdORUdKJmVuY3J5cHRlZElkPUEwNDA5MzEzMTVNRkVDMUxCSlhQRCZlbmNyeXB0ZWRBZElkPUEwNDcwNTYzM0VFVTlSOTJGQVZUOSZ3aWRnZXROYW1lPXNwX210ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU&th=1

Peter,
You may want to check into RC plane fuel quick disconnects. I seem to remember them doing what you need.
Jonathan

Hi Colin and Hermank.
This is a good possibility of the way forwards. Thank you for the help.
A few years ago, one member posted in their blog, no idea which one now, the exact items I am looking for. I added a comment to their blog asking for information on where I could purchase them but had no reply. In retrospect, I wish now that I had sent them a private message with the request. Never mind, I will wait a few days to see if somebody knows about these fittings and failing that, my local fish shop in Budapest will get a visit.
Thanks, Peter.👍😊

Here is the link to what I use.
https://www.ebay.co.uk/itm/324790368338?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=uvN6TlBwRuK&sssrc=2349624&ssuid=LGaRsqMfQ6y&var=&widget_ver=artemis&media=COPY
Cheers Colin.

I wanted to give the same advise as colin did.
i made the dusseldorf years ago and did exactly the same and it works beautifull.

Hi Peter, have the same problem on 3 of my boats, have searched in vain over last few years, so I opted for long pipes so I can lift the tops away and leave connection intact. By adding a push fit pipe connector away from the boat I am able to disconnect without water in the boat. I use connectors from aquarium supplies you can get taps as well so a sealed off section is possible.
Cheers Colin.