The Brew Cabin

I have to construct a manhole under a new extension.
The extension will have a solid floor.
Could someone advise me on what sort of manhole to go for, i was hoping that i could use one of the pre fab plastic ones.

Your local Building Control will tell you what they are happy for you to use. Some will permit a polypropylene chamber: some want brick or concrete.

It's the cover they are normally most concerned with - you'll probably be required to fit a double-sealed airtight cover. You should discuss this with your BCO.