Thanks Colin, much appreciated. Simple is always good for me.
I went for the simple approach as many of the jigs I had looked at involved all sorts of sliding blocks held in place by clamps or bolts with wingnuts etc. This way keeps everything static apart from the webbing strap holding the bracelet buckle.
I've taken some more pics this morning just in case this design is useful for anyone.
The first pic shows the whole jig. The longer section on the left is basically the bit that gets tucked under my backside as I start tying the bracelet. As I get nearer the end of the finished bracelet, I simply pull the jig forward as required.
The overall length is 32 inches and it's marked to allow bracelets of up to 12 inches
(although I suppose you could also make larger sizes or adapt it to make longer items)
This pic shows the top end of the jig. The block was formed by cutting a few sections of a second identical plank then gluing and screwing them to the base of the jig
(you could also use solid blocks instead but this method saved me a lot of joinery work and meant I was putting otherwise wasted wood to good use) Best to attach them from the underside as this avoids them interfering with the attachments for the webbing. I also countersunk the screw holes for neatness
(and so it didn't snag on the couch which would have brought me a world of pain...)
I have a buckle permanently attached by webbing which is screwed in place with washers on the screws to avoid the webbing tearing.
Once you've attached the top buckle, you can measure down before attaching the bottom section. I burnt the graduations on using a soldering iron just for quick reference when setting the bracelet size.
The bottom part of the jig is attached similarly to the top section but also incorporates an adjustable buckle which allows the webbing strap to be adjusted to allow the jig to be set to the size of the bracelet you want.
Hope this is useful for folks. As I mentioned before, it's literally a 15 minute job to make one and if I can do it pretty much anyone can.