I was going to sign up for a computer forum to ask this as its a bit specialised, but though I would let you clever people had a go first...

We are currently changing out a spare room to make a study that will permantly house a desktop windows pc running a double or triple monitor set-up.

What I would like is a KVM switch to enable the use of the keyboard, mouse and screens on a laptop rather than the desktop if required (e.g. photo editing is only done on a macbook pro). Laptops could be a little lenovo touchscreen thing (HDMI) or the macbook pro (thunderbolt/displayport).

I have found a multi monitor KVM but they are silly money. Does anyone know of a more 'economical' option?

What about remote desktopping to the Macbook when required, then no cabling required. Your only challenge might be ensuring the network bandwidth is adequate to deliver reasonably snappy/responsive experience.

I'd go with trying remote desktop or vnc first. - esp as it is the free option.

at work we have vmware guests as our desktops and rdping to it is just like having a pc -- in this case minus 3d hardware capability. but for windows or linux (running xfce) is is plenty fast enough - even across 2 screens. a 100mbit network is plenty, wifi should do it no probs too. We run 10 - 20 users down an ADSL connection.

Where will you do the work on the second machine?

I've got a USB docking station - basically I plug in a USB connection and get two additional monitors to my laptop (plus some USB ports and wired network connection.

Cost around 100 pounds from Dell when I bought the laptop, although i'm sure there are cheaper alternative.

I think the problem with that is that you would need to uncable the keyboard, mouse and screens from the desktop and then plug into the docking station. they are handy for creating a one plug dock, but not to my knowledge for sharing screens /keyboards between different laptops / pcs etc.