I am removing an old oven and need to both disconnect it, and cap off the power supply until I get a new cooker.
If I just turn off the main power switch in my fuse box - it is then safe to handle the cord/cut through it?
Then once I have cut the cable - how do I cap it off? I have a junction box lying around which has some cap type screw things in it - would these work?

Sounds like your afraid.

BTW those cap things are called wirenuts and are designed for specific combinations of wire.

Just turning off the breaker that services the oven makes it safe to handle.

Some options:

Best:
Either turn off main breaker (might be safest for you) or turn off the breaker going to the oven.

Remove both wires going to the breaker.
Cap
Label wire.

Method #2
Turn off power as above.
Place cable in a junction box. There may already be one there.
Cap.

Method #1 is inherently safer and the right way.