So finally had my leaky VW sliders replaced with a set of recessed sliders supplied from THQ/Camper Glass.
I Was having trouble finding somewhere local to replace the windows as it was claimed that replacing the Transporter bonded windows can come with lots of potential issues. The one place who said they could do it seemed expensive and would not tell me which brand of windows they would be using to replace, regardless of me asking a few times. "It's about the quality of fitting, not the brand of window." was their constant reply.
Called the only convertor in Glasgow (who only does full conversions and rentals) for a quick chat about windows and they put me on to their preferred window fitter. Got in touch with this company, popped down for them to have a quick look at the van (which I believe was to check the interior for furniture, etc... that would hinder access to the windows) and they booked me in for a few days later. Dropped off my van at half eight and was back early afternoon to pick it up.
Windows were fitted without issue and haven't had any leaks in the few days they have been in, despite the heavy rain!
Close up finish on the recess (outside rear) top and bottom corners looks a bit poor as it's sealed here as it looks like the plastic frame here joins a rubber frame piece.
Saved well over £200 versus the "wouldn't tell me the brand" experienced window company and haven't had any water in (touch wood) versus the constant trickle down the inside when it rained with the VW sliders.
If the upcoming THQ flush sliders prove to be as good quality as the preview video claims I may look to change to these further down the line, but for now I'm dry and happy!
Thanks to those who read this far and for enduring my rambling.