What's causing this leak?

[GarlicNovember]

And how to fix it? Being super-boring, I've put the full-size pictures here: www.dropbox.com/sh/nfj4gkd2iwmb2zz/AAAaRYsexXo5USTScx-7YenHa?dl=0

It appears to be the cold water feed. It started leaking after I moved the washing machine - I think I must have strained the joint.

Trouble is, I don't know where the stop tap is (new flat) and the plumber who came last week couldn't find it either! So I obviously can't just unscrew everything and have another go.

Can I bung it up with sealant & tape, or is the leak just going to blow through it? Advice on what to do, please!!!

Ask one of your neighbours where their stop tap is or find the water meter and see if it is next to it.

If you've just bought the place, the seller would have been asked where the stop tap is on the property information form they were asked to fill in, so you should have that info. Put a container of some sort under the leak, not a towel if you can.

Good point, Wowfudge, there's now a takeaway container under it ... I'll have to add towels overnight, though, as that little box will soon fill up! It's rented. The property manager doesn't know where the tap is. I'll ask downstairs (after going out for another spanner, as my old one can't get a purchase on the nut at that angle.) I wish I'd left the washer where it was!!

it appears that the compression joint is leaking. You might be able to tighten it up, though preferably loosen it first, push the copper pipe firmly into the joint, in case it has slipped out, then retighten. You have to support the fitting with an adjustable wrench while you tighten the nut, to prevent the fitting turning.

You are lucky that the leak appears to be downstream of the fitting, which is a service valve. If you turn the screw head so that it lies across the pipe, rather than in line with it, it turns off the water.

What pigletjohn said

The slot in the isolating valve is at a slight angle and needs to be straight across to shut the flow off

You will probably get away with either tightening it up a little more or undoing it and wrapping a few coils of ptfe tape around it and tightening it back up again and then re open the slot back to where it is

Is everyone sure it is downstream?
Is that not the feed to the washing machine through the cupboard? Where is the washing machine? (looks like it might be a 15mm pipe - I'm used to 22mm being the main feed - but know that isn't always the case)
You might be able just to nip it up - worth trying but if you over tighten you can squash the olive (brass ring that goes between the joint) -makes it worse! and then you'll have to cut the end of the pipe off (so shorten it slightly) and get a new olive - not too difficult or a major hassle depending on what is on the other side of the cupboard/ further along that pipe - personally unless you are 100% sure that is downstream I wouldn't fiddle with it until I knew where the stopcock is - even if you have to do the one on the street (under a H in the floor -need a key -big t shaped thing -can get from B&Q etc - think they are about £15) and it will be for the whole house - if not a few houses...

I'm glad I asked you lot - you know what you're doing

The washing machine feed is the pipe going out the back of the cupboard. The flexible hose going upwards feeds the cold tap. I've just come back from B&Q with a set of adjustable wrenches, so am going to have a cautious fiddle. I managed to slice my hand along with the plastic wrap on the wrenches, so am possibly not at my most dextrous right now and will be cautious!
Turning the screw to align with the pipe didn't do anything.

if you turn the screw at right angles to the pipe, it will turn off the water supply to the washer. It will not, as I thought earlier, but off the supply to the leak.

Don't turn the nut more than a turn in either direction, unless you can turn the water off first.

I'm not sure this is rational (probably isn't -but would make me feel more comfortable!) - but I think you would be better turning it back horizontal to the pipe (turning it on) until you can turn off the supply ...I know the water will be under pressure at the washing machine end anyway -but if you turn it off at the isolator will that not increase the pressure there - so on that joint?
And I wouldn't touch it until you can isolate the supply - find the stopcock. (scarred by the time I was replacing a leaking bath tap tail and fitting isolators -cautiously turned the mains on, one of the compression joints was leaking slightly so went to tighten it up and the whole pipe fell off ....mains water pouring out and had to climb out of a cupboard, run down stairs to the other side of the house to turn the water off under the kitchen sink - downstairs ceiling still bears the scars in certain lights ).

OMG, I'VE STOPPED IT, I'VE STOPPED IT!!!!

Thanks to two adjustable wrenches - great advice, Piglet, the whole thing tried to twist when I turned the nut. It just wanted tightening up a bit (about half a turn.)

Thank you

Unlucky, I'm glad I didn't end up reliving your story ... but am highly amused by the way you told it!

Well done!
I'm glad you didn't suffer the same trauma ...I really am incredibly cautious now.
Last time I did something I got DP to stand next to the stopcock - using the internal phones as an intercom - I said when I tell you to turn it on just a little and then as I'll tell you when turn it up really slowly, a little at a time.
Told him on - after a few 'ups' I realised a connection had a wee dribble - so said 'off' - tightened it up - and then said 'on again' - checked all the connections - fine - so said right start turning it up - a moments silence and then he said ok. Went round checking fine - right a bit more, check, a bit more -then I realsied I could hear the tv in the background. Sneaked downstairs to find him sat in a chair watching TV! Apparently he didn't realise I meant slowly each time - so when I said on again he had turned it on all the way when he realised that wasn't what I meant he decided it was too late and the best thing to do was to pretend He obviously had more confidence in my work than I did!

Haha, Unlucky, I like his attitude Though yours is far less likely to cause a flood!