I agree. I am trying to set up the 'server' option too. I'm not very technical so really struggling to get it to work…
The annoying thing is in the eCart 4.5.1 release notes:
"Updated to accommodate recent changes made to ProTX/Sage Pay’s gateway"!
If you manage to find a solution, will you post it here!?
If support can open me a ticket, I can send you my php files.