Thanks for your comment. I thought I'd already replied but its not here!!!
I had not installed the minimum Jumbo 25 as in order to do so I would have had to go ahead and ignore the fact I had no link ups and do the 80.40 install, leaving us with no resilience if it didn't work.
In the end I managed to locate 3 network cards that are on the Check Point Hardware compatability list.
I tried the cheap Intel Pro 1000 PT first however after selecting the initial option 11 (install with ACPI support with VGA) the process just hung.
I then tried an Intel Pro 1000 VT and had a similar experience. I then tried both these in different slots on the server to rule out issues there but had the same experience.
With no real hope of success, I removed these and tried the Intel x540 card and it worked straightaway, didn't crash the installer and was visible and showing link up so I was able to safely progress the install. It would have been interesting to then install the hotfixes and try the other cards but I ran out of time on my change window. At least it is all up and running now.