Here is an update on my repairs - in case anyone else is interested in these wagons - they are really nice representations but they have some weaknesses.
1/ Underweight - this is easily solved with additional weights
2/ Badly designed couplings - and here is the reason why. Apart from the problem of the spring (more about that below) the danger area is in when you want to swop over NEM couplings.
BE REALLY CAREFUL WHEN YOU SWOP COUPLINGS. The process will always require some force to pull couplings out or to push them in, this will cause leverage on the coupling assembly causing them to fail internally -and for the wire spring to fall out.
If you can REMOVE the BOGIE easily without damaging the plastic clip, then do so and always support the coupling assembly when doing the exchange.
If the bogie feels like it cannot be removed without damaging the flanged clip, then support the coupling socket with pointed tip pliers.
The coupling assembly with the spring wire is covered by a plastic lid. I have removed the lid and shown the outline in red. This lid can be pushed off from the force of pushing in new couplings into the NEM socket. That means the spring falls out
The lid itself becomes a problem with repairs as it does need to be glued to hold it in place but not so hard that you have to break plastic for the next time you will need to repair it.
In my photo you can see I am experimenting with 0.2mm copper wire - working well so far; however I would recommend 0.2mm springy steel wire which I bought from here:
It is a perfect replacement - perhaps slightly better than the Heljan springs which can become bent like this:
This is another reason why they fall out
A final word of advice - as you can see the spring protrudes either side of the housing for the coupling assembly - this is necessary because it is the sides of the body which hold into place. If the spring is too short, then it will have enough lateral play that it simply falls out of the retaining assembly altogher, if it it too long you won't get the shell back on to the chassis.
Because of all of the things that I mentioned above which have to WORK TOGETHER, there are too many ways for somthing to fail - and so they do
I hope the advice will help along other modellers, as there is nothng more frustrating than hunting down a wagon like this, quite rare at the moment, then find it repeatedly derails and ruins your fun.
I don't expect the new batch from Heljan to address this problem at the current pricepoint - which is why I am taking a lot of interest in the very accomplished looking Kernow IZA Cargowaggons....