There are a few plants with "Robin" in their cultivar name, can you give the full name? It might be Photinia x fraseri "Red Robin", which has red leaves in spring. It's a vigorous shrub, better in the ground than in a pot.

I have got a Photinia fraseri "Red Robin" in a large pot outside my kitchen door. It's been in the pot for three years now and is going strong. It is a very large pot though (one a large box ball was bought in). It is in standard all purpose compost and sorry to say, it's never been fed!. Hope this helps.

The ultimate pot size depends on the variety - there are two versions of Red Robin - one is Photinia 'Red Robin' and the other is Photinia 'Little Red Robin'. As the name suggests, Little Red Robin is a dwarf plant and won't need a huge pot.

If you have the more usual Red Robin, don't put it in a large pot to start with, but pot it on as it grows, till it finishes up in a large pot, so for now, choose a pot that's an inch or two bigger all round than the pot its currently in. As the pot is in the middle of a flat area, I'd be inclined to use John Innes No.2 or No. 3 - its loam based and therefore heavier when wet, so less likely to blow over, and is better for long term pot planting. You can mix it with multi purpose when potting on if you don't want it so heavy as you increase the pot size over the next 2 or 3 years.

Thank you very much, good Goy friends. I have written all this advice down in my notebook.
Will go back to the GC today to get another bucket £1.99
and the J.I. No.3.
I bought 4 of these buckets yesterday to pot my Blueberries on in Ericacious compost.
Like the handle, easier to move when needed.
And the price it right !

I forgot to say - in your description, you say 'red leaves on the ends in winter'. Note that its only new growth which is red - as the leaves mature, they turn green. Pruning a couple of times a year as it ages will induce new, red growth, if its not producing much on its own.