Left it plumbed into the washing machine overnight, and then I went to investigate plumbing supply shops. First into Jewson that I knew sold some of that sort of stuff - but they had nothing to cap the end of it. But they said the Plumb Centre across the road would have stuff, I'd thought they were just a bathroom show room place, but it turned out tucked in the side was a counter that sold bits of pipe & fittings etc. Complete with a somewhat surly shop assistant chap who would probably have reacted OK if I'd been able to ask for what I wanted by name (and probably even better if I'd looked like I was a plumber!), but didn't really know how to deal with someone who had to describe the job she wanted done and ask what bit would work ;) But with a minimum of actual speech he managed to sell me a screw on metal cap for the end of the pipe.
And when I got home it even fit! But then I couldn't get it done up tight enough :/ My adjustable wrench only goes to ~3/4 inch, and this would need a bit over an inch. I managed to use a pair of pliers and get it so it almost didn't drip, but that's not really good enough for long term. I had assumed the washing machine fitters would have a decent wrench & could sort it out for me, but no luck there. I'm gonna go back out this morning and get a wrench and plumbers tape1, if I can (preferably from Jewson coz the staff were more approachable if I can't find what I'm after). And if not, then J gets to go to Focus & get stuff.
However! The washing machine arrived, when it was supposed to. And they took the old one away & fitted the new one (and it went through the door, just, without having to take the door off the frame, too). And I did the empty 90°C wash the manual said one should do first and it didn't leak and did fill & empty the water (and did spin). And the novelty has worn off now, I just want the plumbing fixed so I can return the tumbledryer to the right place & then I can go back to laundry just happening & not being a source of irritation.
1. Or whatever it's called - I wanna say 'teflon tape' coz I'm sure that's what we called in when we used it on HPLC machines in my year out job all those many years ago. HP1090s I think they were, I've not thought of them in ages, but that's what my original adjustable wrench was for (I've lost that one :/ but it wouldn't've been big enough anyway, it was for wee joints).