OK Thanks Guys. I only asked the question as I know that with some interface cables, USB for example, if you intend extending 5 metres then you need some sort of booster (active repeater cable) I believe!?
No, not armchair diagnostics! I have a laptop wired in at the back of my garage with so many peripherals connected to it that it would take me 4 days to disconnect it all! Hence the idea of the longer serial cable so I can keep the laptop in its original position, and the car outside the garage!
If anyone is interested, it is possible to use a serial extension cable with Alfa Diag and the Vagcom adaptor.
I purchased a 10m serial extension cable from eBay (around £6.50 delivered), plugged it into the serial output on the laptop, plugged in the Vagcom adaptor and connected it all to the car, and it works!