I want to add another hard drive to my machine but this would mean I'd
have to use an IDE Ribbon cable with two intermediate pin connectors.
I can't find one on the Internet (Maplin, Dabs), does anyone know
where I can get one?

Each IDE interface can support 2 drives. Should you need to add another, you
need to use a different interface. The majority of motherboards have 2
interfaces integrated on the board, although it is possible to have up to 4,
including the new SATA style interfaces, which only well specified modern
machines will possess.

You need an additional cable and to find the IDE2 (or IDE1, if the first
slot is named IDE0) connector on the motherboard. It will almost certainly
be directly adjacent to the existing slot, but may already be connected to
your CD/DVD drive, with a spare connector dangling around somewhere.

Christian.