Removed kitchen sink but didn't cap pipes...

Moss5

Neither of the service valves are in the fully-off 90 degree position. Getting the slot to align across the pipe may find a fully-off position.
Glad to feel your confidence build with each small success. 

[Deleted User]

To be on the safe side should I get some plumber's tape to go around the joins?

[Deleted User]

Oh yes and finally, is it worth investing in a universal stopcock key, in case in an emergency I need to turn the water off outside?

That plastic handle is supposed to lift off, it's not supposed to actually be used for the turning, right?

Moss5

https://www.youtube.com/watch?v=tbNVu0Qr7_Q
PTFE tape is used on tapered male threads when fitting into tapered female threads.
Many people use it erroneously on compression joints. If you dismantle a compression joint with tape applied, you will see the compression squeezes the tape until it is not effective as a seal.
Your half-inch caps have a rubber disc inside and would possibly seal a leak when finger-tight.
The compression olive has two jobs. The front seal digs in to the pipe to create a barb and prevent the pipe being forced out under pressure. The rear seal just squeezes the pipe to retain any water allowed to pass by the front seal.  

Moss5

My water meter has a plastic handle to operate the 1/4-turn on/off tap. If you are unable to turn the water off, ask your water supplier to provide a tap and handle. 

GDB2222

Moss5 said:

https://www.youtube.com/watch?v=tbNVu0Qr7_Q
PTFE tape is used on tapered male threads when fitting into tapered female threads.
Many people use it erroneously on compression joints. If you dismantle a compression joint with tape applied, you will see the compression squeezes the tape until it is not effective as a seal.
Your half-inch caps have a rubber disc inside and would possibly seal a leak when finger-tight.
The compression olive has two jobs. The front seal digs in to the pipe to create a barb and prevent the pipe being forced out under pressure. The rear seal just squeezes the pipe to retain any water allowed to pass by the front seal.  

I've always used this sort of stuff :https://www.screwfix.com/p/fernox-jointing-compound-400g/21548

It's easy to apply, and it works. It may not be necessary, but it gives me a feeling of comfort.

[Deleted User]

Thanks for the further answers! 

A final question!   I noticed while doing this that the smell coming from the open waste pipe isn't so fragrant, and I can figure out why!

I've now temporarily capped it with a binbag and selotape.   That'll do till I get a kitchen-fitter here to fit the new sink.

Now just to paranoically check - sealing off the waste pipe isn't going to lead to some kind of subterranean methane explosion, is it?   It's just an obvious thing to do it?

GDB2222

Yes, it's fine. The waste system should be ventilated somewhere.

molerat

The waste pipe will be full of smelly stuff.  A plastic bag taped over it is fine.  Just be aware that, depending on where it fits into the waste system, a washing machine emptying could cause water to come out of that sink waste.

[Deleted User]

Molerat, there's just one waste pipe on this level.  The washing machine and sink were both attached to it.  Fortunately I've had nothing but bad smells coming out of it! 

It's an old Victorian house converted into flats.     There's a washing machine on the floors below, but I think that'll just go downwards.

Thanks again all for your help!