Unidentified flowering shrub/small tree.

It's a California native as far as I know. Not sure of the name but it starts out small but can grown to about 15 to 20 ft. The booms in Spring are beautiful. Google CA native shrubs with yellow flowers and you might find it. Might have a look myself. I'm thinking Cassia.
Ok I was nearly right I looked it up.

Yours is a type of this.
Your flowers are different.

Acacia, golden wattle, green wattle or western coastal wattle (Acacia cyclops, Acacia dealbata, Acacia decurrens, Acacia melanoxylon or Acacia longifolia)
INVASIVE — DO NOT PLANT — INVASIVE

Invasive or not I still like it.

http://www.cal-ipc.org/landscaping/dpp/photos/Shrubs/acaciacyclopsClose.jpg

Invasive??????? I do not like the sound of that. I had a look around it earlier, braving galeforce winds to do so, & I cannot see any spread of it. It just looks like a shrub or small tree at the moment .... probably around five feet high. Mind you, not much chance to spread as it is hemmed in on either side by other shrubs. Interesting & I do thank you for going to so much trouble. I've been visiting garden centres over the past three weeks, & I have been looking out for it, in order to see what it is, but haven't seen any sign of it. Very pretty, whatever it is. I will leave it alone & see what happens in another couple of years .... I have a native yellow kowhai tree (now about 7 feet high) just a few feet along, which my husband has informed me will grow to around 15 or 20 feet high, so could not cope with yet another huge yellow tree along the driveway. lol

This type you can hack back and keep it to the size you want. I think it might mean that the seeds from the flowers are easily germinated so it spreads fast in the wild. In your garden you can dead head before the seeds spread or save a few and pot them up to see what they do. Mostly means it hardy too.

Phew .... that's good. I will be vigilant! Thankyou so much for your help in identifying it.