There are different sizes and for some it may be necessary to lengthen
the hatch space. Basically you take out the existing hatch assembly
and replace it with the integrated ladder unit

Thanks for the link - I'll get the ladder out and measure up the existing
hole, which just looking at the web-site picture isn't long enough. But at
least it can be lengthened quite easily. Luckly we're redecorating the whole
house so making a mess in extending isn't too bad.

Cheers, Rob.