We have had our cellar converted, and the builder put in one of these

http://www.triton-chemicals.co.uk/prod4.php

Its been concreted in place, and our builder has layed chipboard, and then plyboard, ready for flooring. He has cut a square shape out of the board so you can lift it out to get to the pump etc.

I intend to put 600 x 300 slate tiles down, so my thinking is that I need some form of manhole cover (preferably hinged) so I can get access when needed, but I want to be able to still have the area covered in the slates.

Where should I look?

Ta

you will need a sealed recessed tray cover. Get it from local builders supplier or google it

What you need is called a floor access panel/cover. Googling that will find plenty, but a pro tile supply place will hold stock.

Like this?
Look around here for the one to suit your subfloor depth

Yep those are exactly the type of thing we need!

Any ideas on cost ?

haven't you seen the Great Escape – like they did in the shower?