I have read a little about the National instruments package which from what I can understand offers control and logging options from connected devices but as with most things it comes at a cost, quite a considerable one at £1200. Definately not for the home enthusiast.
Since GPIB isn't "modern" anymore, buying new interfaces, software, etc. often will not be cost effective. It's something you want to get used. Often folks will get a USB- or Ethernet-to-GPIB adapter to connect older GPIB equipment to newer computers that lack ISA or PCI support. Just be aware that there are also counterfeit (branded with NI, Agilent/HP) and clone USB-to-GPIB interface devices out there.
Also are older machines supported still or is it a generic set of commands used? I have read about third party software packages, but they seem tailored to specific instruments or manufacturers.
Each device has a command set. Whether a particular software package supports a particular device is something you'd have to check in the specifications. Often, there are drivers to support specific devices, written by the control software provider, the manufacturer of the device, or a third party.
The PCI GPIB card itself is not a particularly cheap item and then throw in the cost of a few cables it all mounts up. Is there a cheaper/freeware alternative for the software side of things, If I decide to go ahead it will be with an older National Instruments card as it appears to have more software support.
From what I've seen, most people seem to write their own code to do what they need with the instruments.
Another driving force to the enquiry is the recent purchase of a faulty spectrum analyzer, the display side of things has failed but everything else on it appears fully functional, would GPIB allow me to control/monitor what is going on with the device as part of a diagnostic process.
It depends on the device, but yes, GPIB is a useful way to test the functionality of an instrument especially if there is a problem with the display or controls. An example of this kind of usage can be found in some of The Signal Path videos on YouTube where Shahriar connects a broken device he got on eBay to see if it's alive because the front panel is dead.
I have done quite a bit of Googling on the subject but I thought I may get more sense from someone on the forum that uses the system..
As rstofer points out, GPIB has uses, but not everyone needs it. If you find that you occasionally need to log many values over time, test broken gear, or some other control function that GPIB provides, you can get one interface and one cable rather than connecting everything full time.