Oxalis triangularis "Cupido"

This is definitely a plant that should not be grown here. It takes over very quickly and becomes a tremendous, noxious weed. Most likely you don't have that problem there in the UK, right?

This species is fine (borderline hardy), but other species are problematic.

I think all Oxalis species have a 100 percent germination rate of their seeds.

That I would not disbelieve! Sadly, this rarely sets seed.

Here it would set seed. They all set seed here in California.

I've had some kind of Oxalis growing in one of my plant pots in the greenhouse it was trying to push the other plant out I've no idea where it came from, I carefully pricked it out and have transplanted it to another pot of it's own to find out if it will flower the leaves are huge ...

Has it got bi-coloured leaves Amy?

Yes it has , I've Googled it , Oxalis tetraphylla ' Iron Cross ' nothing special about that then :o(

It's a nice enough hardy species. Can be a tad thuggish though..........

I don't think I'll plant it in the garden ,I guess the birds brought it in !

Or the MPC that you use - the rhizomatous roots are supposed to be pretty hard to kill.