Saw this today at RHS Wisley - think it's called Oxalis triangularis (Purple Shamrock).

Yes I agree :)

Yep, Oxalis triangularis - was outside today tidying mine up and just noticed its got rust... This plant is frost tender I'm afraid, so you may need to dig it up and remove to shelter for winter, living where you are. I leave mine outside, but its in a sheltered spot against a wall in London on a south facing balcony, so it survives.

I posted on the first post that has been deleted, but can also confirm that it is oxalis triangularis. My plant pack also calls it the Love Plant. Mine is in the ground and comes up each year but we've not had any really hard winters yet. I may move it to a pot as the foliage doesn't really stand out against soil - same with the black Orphig... Grass.

That's true, Barbarak, dark leaved plants don;t show up well against soil, very annoying... regarding winter survival, some of mine appears to have migrated 3 floors down to the garden downstairs (no idea how) and pops up by end of spring in various places, so its survived in the ground - but its a risk in colder regions if we get a proper winter rather than what we've been having (a sort of extended autumn instead).

I've got this plant too! Mines in the ground and has resurfaced from last year.. But it has now got rust!!

Yep, common on these, mine's got it too... I;m ignoring it, won't be long till the leaves disappear anyway