I'm looking for a new lightweight waterproof jacket that is as breathable as possible.

I am about a size 12-14 and have a bigish bust 34DD and a big bum that it needs to fit over. I also want to be able to wear it in the winter with a good base layer and fleece.

I have had a look at the Gore Phantom that is in the sale at Evans at £65, but have heard their sizing can be small and I wanted to try and spend a bit less than this. 

Any recommendations are welcome.

Don't know about the Gore Phantom but the Gore Alpine is my suggestion. It's expensive but very, very waterproof and very breathable. I bought one on Trio's recommendation having tried a couple of other cheaper jackets that all seemed like they'd be waterproof and breathable but didn't quite do enough for me. I have had so much use out of the Alpine and never regretted the huge chunk of change it cost. If I lost/destroyed mine I simply wouldn't hesitate to buy another.

Size wise Gore stuff is fitted but not tight like some manufacturers, if you're a 14 then a 14 jacket will be right. 

As Jumbly says the Gore Alpine is a great Jacket, loads of us on here have one.  Mine gets lots of use and is still going strong!  I have a Gore Phantom and it is a softshell which although showerproof isn't waterprof, it is a great jacket, the removal sleeves get my vote, but doest replace a waterproof, not in this country anyway!

Evans have it in a size 14 in the sale

http://www.evanscycles.com/products/gore-bikewear/alpine-lady-jacket-ec011718

I find eVent jackets much, much more breathable than Goretex!!

I normally these days though wear Paramo - wash in TX Direct and they are totally waterproof but completely breathable - and as a cycling insructor I'm out in all weathers!  Wouldn't go back to Goretex ever.