I fitted a +0.30mm oversize locking latch from gunspares in the end. Worked a treat.
Not knowing which to get - 0.15 or 0.30 I first looked at the release lever position with no barrels attached and 'released' by pressing the small release pin, then again with barrels attached. Both were the same position. With barrels attached a small gap was visible at the top of the barrel mating face that could be closed by squeezing the gun shut (but no gap from twisting motion, so not loose hinge pins)
I then made up small shims out of acetate measured with micrometer and fitted these into the mating holes in the barrel faces, adding layers until the locking lever began to sit right of centre again with the gun closed. I measured about 0.25mm thickness needed, so went for the 0.30mm.
The new +0.30mm Latch has a number "2" stamped on it (annoyingly on the side that is hidden when it is installed).
My previous loose latch when removed has a number "1" stamped on it, and signs of previous work (and very worn mating holes on the barrel face itself) so I interpret the "1" to be the +0.15mm first oversize. My shimming method was therefore not to be relied upon, as I only apparently increased the size of the lugs up by 0.15 from the already oversize lugs fitted.
#1 locking lug (presumably +0.15)
lug end tip minor diameter (on flat faces) 4.18mm
end tip major diameter (on round faces) 4.93mm
#2 locking lug (known +0.30mm)
end tip minor diameter (on flat faces) 4.30mm
end tip major diameter (on round faces) 5.16mm
My approach was below for interest, but use at your own risk/don't do this yourself etc etc.
Stripping the action down as far as the locking lug was straightforward, but getting the lugs out is a PITA, and for this alone I would just give the gun to a gunsmith.
Using a Youtube video: "Episode 165 - Beretta 68x Detailed Disassembly". Don't disassemble the trigger assembly though. Referring to items on this schematic (click part numbers on the schematic to get zoomed in picture)
Cock hammers. Remove barrels and stock, drift out one small pin #52 holding the safety/barrel selector assembly and lift out. Unscrew trigger group top screw hiding underneath safety #54 . unscrew small grub retaining screw#78 and large screw#55 by trigger guard (used extra-thin ground down flathead bit). This is as far as the video goes.
Drift out the small pin #86 holding the safety lever gate block #101 and remove it, don't lose the spring behind it. Mine is not auto-safety so yours may differ.
I took out the top firing pin #40 and spring #42 (drift out retaining pin from right to left - then the firing pin will be freed up first, then be ready to catch the locking latch #38 and spring/guide #36 #37 that is also held in by the same pin. Remove the locking latch release pin #35 as well so you don't lose it.
You can then get better access to the top lever retaining nut #33. To do this I used a security flathead screwdriver (with a notch cut out the middle) to remove the nut #33. The centre threaded pin that screws into the lever itself #34 I left in place (here I saw evidence of previous galling from a poorly fitted screwdriver as mine had a slotted head and was seized tight, but I see the newer design has an allen head to this small screw #34. Once the retaining nut is removed now comes the tricky bit of removing the 'top lever pin' #32 - the lever must be opened slightly (as if opening the gun) to allow this to clear the locking pins (one edge of the top lever pin is obscured by the locking pin crossbrace). to pop out of the mating hole in the top lever itself. Rightly or wrongly I did this by wrapping the block in a kitchen towel to catch flying springs, holding opening the cocking lever enough to see daylight past the face of the top lever pin then drifting the 'top lever pin' #32 down into the gunfrom the topside of the lever with a wood dowel. A few sharp taps and it came out. You can then withdraw the locking levers and the lever return spring and plunger #30 #31. It's a three-handed job but doable with two with some effort.
Refitting the locking lug was exceptionally fiddly as the lever return spring is under great tension. I read elsewhere that a jig can be made - presumably to compress the spring using a thin flat plate and clamp flush with the rear of the block whilst you refit the top lever pin. I couldn't get this to work.
What I did was Refit the return spring and plunger #30 #31 and loosely fit the locking lugs#29. don't press home all the way, as you now need to drop the top lever pin #32 into place - the cam on the top lever pin #32 that mates with the locking lugs can be slightly engaged, but note that the top lever pin won't sit flush with the face of the locking lugs, and won't fully engage, and the arm of the top lever pin will only just engage with the return spring - it fits underneath the locking lugs only when the top lever pin is engaged in the pivot hole in the block that the top lever sits in.
To get the top lever pin into its hole I got a quick-grip one handed bar clamp with soft faces. One face rests on the receiver face where the firing pins stick out of, then other one only just grabs hold of the back of the locking lug by the block tang, enough that you can squeeze (with some force) enough to compress the spring so the top lever pin gets close to its hole. I used a drill shank poked through the top lever pin hole from the top to guide it home, with light taps from any face that needs it, all whilst making sure the clamp didn't slip off the back of the locking lug. Once the top lever pin is over its hole, tap it home and it will eventually sit flush with the face of the locking lug, and then the locking lug will click forward and partially cover it. You can now remove the clamp, refit the retaining nut and re-assemble everything else in reverse order, noting and renewing threadlock where used.
Whole process took me 2 hours the first time - mostly trying to get the top lever pin back home against the spring pressure, but then I disassembled again to take measurements and the 2nd time took under 30 minutes start to finish.
Lever now sits slightly right of centre when gun is closed and gun locks up with no gap at the barrel face opening up under flexing.
If I wasn't needing my gun the next day I would have used a gunsmith, but I'm glad I did it myself as I am more confident in cleaning the internals.