potential dumb question but i just thought i'd check, you're using a presta valve, is the rim drilled for schrader?
assuming it is drilled for only presta, you could try winding the nut to the base of the valve then fitting the tube and putting another nut on top once the tube is inflated.
Yes it's a rim that's drilled only for Presta but it's a really narrow rim (Sun M14a) with an internal measurement of about 12mm which won't leave me a great deal of space for the tyre beads with the nut inside the rim.
Make sure there is no sideways pressure on the valve caused by pulling tyre around the rim as you mount the last bead too!......
When fitting the tyre i usually push the valve as far in as i dare to get the beads on the rim first as it's quite narrow then pull the valve down a little... gently i might add so it's seated ok.
Had this before myself are the tyres new or old? I found my older tyres were moving on the rim and dragging the tube with it and tearing it on the valve hole in the rim....also the other things mentioned talcing, lock ring to tight, and low pressure....
They are quite old.. I bought the tyre in question in the mid 90's but it doesn't seem to move on the rim mate.
As with my OCD the valves are lined up with the script on the tyres
Simply crap inner tubes. Cheap is often not cheerful; Schwalbe or Conti are dependable.
You could be right there bud and have been looking at the Conti lights on eBay and may try them but they're costly and want to make sure i resolve this first. A tenner a pop.. quite literally.
Stupid question, but have you got rim tape and is it correctly applied / not peeling off?
It's certainly not peeling off but there is slight movement.. Going to remedy this now with a bit of insulation tape.
If the rim is drilled for schraeder valves and you are using presta valves, a fix that has worked well for me over the last decade or so is to cut out the section of an old destroyed tube from around the valve (ie where the tube has failed) and put this section of rubber over the valve on the new tube before installing it. This gives a decent amount of protection to the valve/tube join on the new tube for little, or zero, expense.
Liking this idea Graheme.. Liking it alot.
Far too many old Marins...