Ok job done. Details as follows for anyone else who comes across this thread. This should work for adding an OCXO to a Racal counter. Bliley module was £13 from Aliexpress.
I reused the analogue to TTL input circuitry and bypassed the doubler, stealing part of it to provide a stable buffer for the doubler and the main counter IC. This involved snipping out a 7400, resistor and capacitor and soldering a jump wire.
The existing module had an interface compatible with the new module but the new module needed a trimmer adding. Decided to knock up a PCB for this. Rather than wait for China I used a bit of FR4 and just hand drew it out for etching. Took some thinking and measuring then the holes were drilled then it was cleaned and then traced out.
Into a sink of boiling water containing a tupperware tub full of ferric chloride. Yum. Poked around with a cocktail stick low tech style.
Out, cleaned and tinned (tinning isn't great as it was done with an iron and solder but it'll do):
Populated. The trimmer is on the back so it's available in the "calibration hole" in the back of the unit.
Now the board was modified. IC4 was removed and pins 1-6 jumped. C36 and R60 were removed. This effectively bypasses the doubler.
Board then installed.
Does it work? Woohoo it did! Went through careful cal after this and is bang on the mark now.
Quick test with the MFJ doohickey at 150MHz to check channel A.