You may not need decent wet weather gear, but if you do need it, you will really need it! What I am saying is that if yo are lucky with the weather, you will be fine, but if you are unlucky, you will truly wish that you had good wet weatehr gear. There is nothing worse than being cold and wet for days on end, which, while unlikely, is possible if you haven't got good gear.
On balance, I would advise taking decent gear. It is not necessary to buy the top of the range Henri Lloyd gore-tex stuff for a squillion dollars, some of the less glamorous brands are just as good and a fraction of the price. I would also suggest having a good look on e-bay. There are some very reasonable prices on good wet weather gear (try searching for "Musto").
I would advise taking
a) Decent wet weather gear
At least one decent polar-fleece jacket
c) A p[air of polar fleece pants
d) A set of thermal long-johns and top
Sure, you might not need them, but if you do, you will be grateful.