ADF4355-3 failure to lock and general questions

I am trying to use ADF4355-3 as an upgrade for an existing product that successfully uses ADF4350.

I do not have the evaluation board, but I am using ADIsimPLL to verify some register settings and my loop filter

I am pretty sure I am communicating SPI correctly and am programming registers in order from R12 down to R0 with short delay before R0 as specified in data sheet.

The PLL is outputting RF at approx the frequency I specify but the tuning voltage is all over the place and the digital lock (on MUX) is usually always low, sometimes it randomly goes high then low again. I am getting plausible waveforms from MUX when set to Ndivider or Rdivider. All this makes me think the problem is in the PLL feedback loop.

I have of course checked and re-checked my hardware and my SPI registers values and can fin no fault, and have been working on same for many days without success.

I have tried twiddling some of the register values with no success.

Just once I thought the PLL was locked - digital-lock was high and Vtune was stable, but before I could check with a spectrum analyser it had returned to random again.

I must say the data sheet (rev B) and ADIsimPLL are far from satisfactory. Sometimes the same entity is referred to by different names which is very confusing. For example in ADIsimPLL under "Time Domain" / "VCO Autocal" there is an entry "ALC Timeout" but I cannot find any reference to this elsewhere so I do not know what to do with the value it suggests.  I cannot find any applications notes that might walk me through a trial setup. And I have checked all the EngineerZone posts relating to ADF4355-3 and cannot find any answers that help, and note that there are several unanswered questions which also is not helpful.

I could do with some help please!