I have a partial answer to my problem; in that I know what the issue is, but I don't know how to solve the issue.
The problem is related to the problem with the DP55 LAN which there have been several threads about in the desktop section of this forum in that the LAN runs at 10Mbit even when the switch it connects to is capable of more.
I can force the switch to full speed and TFTP then runs at the correct speed; but because of the machine production environment I cannot leave it on anything other than auto as other LAN controllers on different motherboards then fail to behave correctly.
I have yet to find out how TFTP decides its link speed and if/where I might change that; so if anyone has come across this that would be great.
I have been able to use the new 14.6 driver for all the network logins beyond the initial TFTP load; although LiveXP takes 7 minutes to load I can access the network at the correct speed within LiveXP, so although Intel have 'fixed' the DP55 LAN problem with a driver update I suspect there is a lower level issue perhaps with their PXE firmware.
My guess about the firmware appears to be correct; the newly released bios which contains a PXE firmware update has fixed the problem