Yes same as yarrow. Depends on what you want as to if you keep it or not. Can't be sure but looks similar. Maybe pot it up until you definitely know what it is. I find them fairly pretty and yes the wildlife like them.
I'd get rid of the clump of Spanish whilst it's still only a clump aym - as holly hock says - they bring nothing useful to the garden and are an invasive threat to native bluebells.
I envy you your yew tree
I have just planted up some yarrow in the front border hollie hock - I love it for the pollinators it brings amy - it might not be the common yarrow but one of the perennial Achileas - I have a cerise one which is gorgeous and very heavily occupied when in flower
Yes You don't have a big garden and they will eventually encroach on everything - they are coming up through my front border - through poppies, sedums (the big ones), polyanthus, primroses - you name it and they're coming through Theses are from next door who doesn't 'do' gardening .......
Very sure aym - I have muscaris over the other side and at the bottom of the border for the early bumbles
I don't think the leaves you are showing in the pics are wide enough for Spanish bluebells however - are these from the clump you're talking about? When they flower, native bluebells have their petal tips curved well back and usually have creamy (maybe greeny coloured pollen). Spanish bluebells have wider leaves, the petal tips don't curve backwards (a lot) and usually have blue pollen. They don't have any scent whereas native bluebells do
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Could the second one be a bluebell or a spring onion? They look similar.
I'd get rid of the clump of Spanish whilst it's still only a clump aym - as holly hock says - they bring nothing useful to the garden and are an invasive threat to native bluebells.
I envy you your yew tree
I have just planted up some yarrow in the front border hollie hock - I love it for the pollinators it brings
amy - it might not be the common yarrow but one of the perennial Achileas - I have a cerise one which is gorgeous and very heavily occupied when in flower 
2 and 3 do look a bit like Snakeshead fritillaries - the way the leaves come off the main stem as it goes up.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Yes
You don't have a big garden and they will eventually encroach on everything - they are coming up through my front border - through poppies, sedums (the big ones), polyanthus, primroses - you name it and they're coming through 
Theses are from next door who doesn't 'do' gardening ....... 
Very sure aym - I have muscaris over the other side and at the bottom of the border for the early bumbles
I don't think the leaves you are showing in the pics are wide enough for Spanish bluebells however - are these from the clump you're talking about? When they flower, native bluebells have their petal tips curved well back and usually have creamy (maybe greeny coloured pollen). Spanish bluebells have wider leaves, the petal tips don't curve backwards (a lot) and usually have blue pollen. They don't have any scent whereas native bluebells do