I'm also using single output LNB's - Octagon and other types - modified for LO injection through the downlead coax cable. I was puzzeled by the amount of LO power needed for the PLL to lock. In my case something like +3 dBm to + 7 dBm. It turns out that with single output LNBs, where the 25 MHz shares F-connector with DC supply voltage, the LO signal is loaded down by a capacitor - 0.1 to 0.33 uF - placed at the input terminal of the 78L06 regulator. Only a meander line RF-choke prevents a total short. This circuit is used on all LNB I have seen.
My solution was to put a 10 ohm SMD resistor in series with the meander line. It reduces the loading effect of the 0.1uF? capacitor and increases LO injection sensitivity by 15 dB. So now only -10 dBm is necessary. It also gives a 1 V voltage drop, but with 12 V supply to a 78L06 this is not a problem.
The LNB is not an Octagon, but a cheap 7 Euro PLL type bought locally.
Diplex filter in the shack to separate IF, 25 MHz and 12 V DC is homemade. Just a few components in a metal box.