Canyou clarify what you mean by mimosa? People sometimes use mimosa to describe mahonia aquifolium, but more often it refers to Acacia dealbata - but then there's Mimosa pudica ....
Assuming its Acacia dealbata, this one is frost sensitive - you're likely to lose it in an exposed position, or in a pot, when temperatures fall below -5 deg C. Where you are, I'd imagine that happens frequently - we've also had some chilly weather in the last few weeks, so your young plant might be reacting to that rather than what you've fed or not fed it with.
Is it in a pot and what have you been feeding with? If you're still feeding it, stop immediately.

oops, I see what you mean, Bamboo, just shows you how inexperienced I am! It's actually a Albizia Julibrissin Rosea, or a Persian Silk Tree but everyone says its a mimosa! Hope this means anything to you!

Do you think I should bring it either indoors or put it in the porch? I haven't fed it (miraclegrow) for weeks and yes it's in a pot but a big one, it's got plenty of room..

Do you think I should bring it either indoors or put it in the porch? I haven't fed it (miraclegrow) for weeks and yes it's in a pot but a big one, it's got plenty of room..

Ah, different plant then. The one you mention is the hardiest form of Albizia, so that's a good thing. Is deciduous, so will be dropping its leaves from now on, which is probably why you've got some browning. If planted outside, needs to be against a sheltered fence or wall, preferably south or west facing. Normally I'd suggest you placed it in such a position for the winter in its pot and just bubble wrap the pot - however, I've seen the possible forecast for the winter this year, lol! I think I'd plan to move it out of the weather when and if it gets really bad, so think about where you can keep it that's very cool, but not freezing. As its in a pot, it's more vulnerable in a bad winter.

According to Burncoose Nursery's website Albizia Julibrissin Rosea is only hardy down to -5 degrees, so you'll need to put it in the porch when the weather gets colder, and possibly indoors (somewhere unheated) on really cold nights. If the porch is a sunny one the temperature in there will get quite high on sunny days, even if the weather is cold outside and the temperature variations will be very hard on your tree. Somewhere with a more even temperature would be better.

This tree is deciduous, so probably the brown margins on the leaves just mean it's getting ready to shed them.

Oh thanks so much you two, that's great advice, I didn't realise that it was more vulnerable in a pot! I thought it would be better, but I honestly don't know much (but I'm trying to learn). I will absolutely take your advice. So it's not a mimosa atall then?!

No, it's not.

I'm not sure what the problem is in the UK, but here in the USA, it is known to be hardy down to -20º C. Still more vulnerable in a pot, though, and still needing lots of sun and warmth in summer.

It's probably our deadly combination of damp and cold that does it, Tugbrethil. Does for my arthritis as well...