After some playing around and a lunch break I'm now able to ping servers on my test network by name when connected to the SSLVPN using the SSLVPN client v4.0.2317.
After upgrading our test FG60D to 5.2.4 to test some outstanding issues that are already open with support I noticed on comparing the before and after configuration backups that the portal's split-tunneling-routing-address went missing during the upgrade. On attempting to reconfigure it it became apparent that an address range (for some servers on our DMZ) was no longer accepted. Removing this address object and replacing it with the DMZ subnet (splitting routing for internal and dmz subnet objects as total) was successful.
nslookup still only lists my standard connection DNS server however the issue seems resolved for me.
I will be making the same configuration change on our live FG (running 5.2.3) to see if that resolves the issue with our live VPN.
Change now in on the live FG, issue seems resolved for our configuration.