Latest instalment in my quest to overcome mechanical twattery. Well after a week of building and rebuilding I can add VW lock expert to my brief skill list and can strip and rebuild one in no time. One thing I'd mention here is that some ebay breakers will sell the same part with different part no. searches i.e. you may not get the part you think you're getting ("They're all the same mate"). One lock I got, which turned out to be for a Seat, did not line up with the mounting bracket, and another seller even supplied me with a lock without electronics to fit a vehicle without central locking. They will also carrying on using the same photo too. All put right but added more time and messing about.
So I rebuilt a drivers lock with all the best bits from each and tested (with the door open
@Loz 
) and again it worked perfectly opening and closing, until I locked and unlocked and then tried to open, at which point the same happened again! So lock out of the van and after much experimentation I've finally found the problem. It wasn't deadlocking, it wasn't the alignment of the latch, and it wasn't a faulty lock. What happened was that at some point I'd managed to pull the end holding clip of the internal release cable further towards the end. Less than 10mm, but the T6 has much less play in this cable than the T5 to give a more precise action, and that 10mm is enough to prevent it from moving the opening lever over the full range. When you lock the door from the inside using the door button, you are pivoting that plastic lever at the top down to stop the metal unlatching lever moving up and opening the door. When you then pull the internal release, the cable pull the metal lever up, which moves the plastic lever out of the way, unlocking the door. The second pull then opens the door BUT ONLY IF THE METAL LEVER RETURNS TO ITS LOWEST POSITION. If the lever cannot go all the way back down, it will not reset and then will not open the door, no matter how much you pull it, and you are left in the limbo of having an unlocked door that will not open from either side.
So once I'd realised that, I worked the end clip further down the cable (it's very tight, god knows how I'd moved it) so that at rest it now looks like this
That made all the difference, the latch and lock barrel are now back in the driver's door and it opens, closes, locks and unlocks flawlessly every time.
So a problem solved and a minor victory, but it only gets me back where I started. I now need to do the same on the passenger side when I'm next off, and then try and establish if the sliding door needs a different lock (it probably does given that my original front door locks didn't have the deadlocking mechanism) and source one. It's then going to be a matter of working out how the recoding is done, and whether the door control modules need replacing or if the BCM controls them.
