Anyone know?

Could be Agapanthus.

What j52 said.

If they are, they do best by being overcrowded in the pot! It put them back in the same pot with new compost and put in a frost free corner.

I don’t think they have to be frost-free, J52. I have a big clump that has been undisturbed for the last 20 years in my cold clay-soil garden & flowers every year.
Maybe some varieties are less hardy?

J52 these have never flowered to my knowledge ( I don’t even know where they ve come from or when) just remember seeing the pot full of greenery for the last few years there really was dozens and dozens in fact I ve got a big old pile perhaps I ll put them in the garden where they ve got more room to spread if they want to and see what happens

I have 2 clumps in my clay soil, one near a hedge which flower poorly and others in a more open position which flower really well. They are both better since I needed the pot for something else and thought I'd try then in the garden!

What is a frost-free container please?

Some Agapanthus are liable to frost damage and if container grown need to be protected from extreme cold. Easily done by covering with horticultural fleece or popped into a sheltered corner.
They do well in the garden in moisture retaining soil. They do need summer sun to flower, so an open aspect is best.
This year’s sun has been great for them.

These look like dahlia tubers to me.

Are [plants in containers more at risk of frost damage?

Yes, the pot gives little protection to the roots as they are all close to the edge of the pot. Wet compost freezes close to the roots, all around the pot.

Ground planted roots are protected by soil layers and frosted soil tends to be on the top.

I have just put a "boxberry" into quite a large concrete container. It says it is hardy down to -10 degrees. Not sure whether I might have done the wrong thing.

I think it would be ok. Just watch out for extremely cold weather. If it’s forcasted, wrap some bubble wrap around the pot.

I was going to say dahlia's too.

Iris's ?

Gladioli's ?

I think that they could be Crocosmia. They very soon become overcrowded and stop flowering.
If you plant out and they stop flowering eventually, then you have to break up the clump and they will flower again.

No idea, they look a bit small for agapanthus ( although if they are, they like being crowded in a pot) so, how tall do the green stems grow?

Sorry just had another look, they're not Crocosmia.
Whatever they are will be a nice surprise next year!

Could be nerines?

Dahlias I think, definitely not frost hardy!

Hi J52 - so I wrap it around the pot rather than the plant itself? The pot is a sort of concrete low urn shape about 18 inches square.

Yes the pot. It will act like insulation.

Would also say dahlias..