Lysimachia punctata and a thug of the first water where suited. There are much better yellow flowered plants which do not come with a Health warning.
Have a look at Coreopsis or Heleniums or the perennial types of Rudbeckia.

ok thanks for the warning!!!

Lysimachia, yes, common name Loosestrife - I still grow it anyway, I just dig out and split when it spreads too much, usually biennally. Good in shady damp spots too. Coreopsis grandiflora is great, but can be shortlived, so might need replacing more frequently - but its in flower for months.

Hi, this is my 1st summer in my garden and had asked for a ID on this plant as it was everywhere! It had been left to its own devices and taken over. Jen

If you have room for a small shrub Potentilla fruticosa is good - several different shades of yellow available and flower all summer.

Welsh Poppies brighten up my garden, and yes they do self-seed but are easy to pull up.

My suggestion would be Buphthalmum salicifolium, a reliable clump forming perennial which doesn't seed around. Lots of lovely yellow daisy type flowers.