Ok I have not trained for well over a year and a half, but i might go back tomorrow as my daughter went back a few weeks go and has got straight back in to it, as if she never left, I do not like to just sit and watch.
The problem is when I trained I held a brown belt but now I am not sure I will remember a lot of the katas. So should I wear a brown belt straight away?

I would think the best bet is to speak to your Sensei. He may prefer you to be beltless for a while or something until you get back up to speed.

Within a few weeks of joining the wado ryu club I'm in, I'd learned five katas... Should I be instantly graded to blue belt?
Indeed not!

The kata's do not make you the karateka. But your abilities do. You reached your current grade through ability and experence.
Should a black belt wear a kyu grade belt until he can prove he can do the katas, just cause he was ill for a year?

Look mate, you earned your grade. Wear it. You'll soon pick the kata's up again.

As a Sensei, I have people training with me who have reached higher grades and then stopped.

In one case the student trained with me a few times and, after speaking with my Sensei, I told him we were happy for him to wear the colored belt he had previously achieved. Other students just wear their belt without issue. It does depend a bit on how long the break has been, but generally, as long as you trained in the same style, you should be able to wear your belt. This enables you to stand with other students of that belt level and pick things up that lower belt students don't know yet but that you would have done before.

In any case, safest way is to talk to your Sensei. Don't be surprised if you pick things up quickly. Welcome back.

Barbah