Incase anyone wants to know I eventually found the answer to this! Believe it or not it actually came down to NTFS permissions (relevant to filename/folder length how??)
The problem was on the ROOT of the share being used, someone hsd stripped back NTFS permissions to certain users. eg
"programname is not a valid short filename" etc, and you check all the permissions in that folder and so on and wonder why it works fine on XP 32bit but not win7 64bit.
The reality is up at the apps level, you need to give either users or auth users read permissions - I found 2 different shares using different tickboxes under advanced permissions but the end result was equivilent I think to basic reading capacity.
Anyway once we did that, we found we cuold install said software. There were plenty of other solutions offered by people but none of them worked or were relevant. This one was a TOTAL waste of many hours of work mainly due to the completely red herring nature of what little information you are given. Another lesson learnt was always be aware of what permissions people have set on your share roots!
Anyway hope this helps someone somewhere