Iron on/in some wonderweb would be my advice but yes chops will know or one of the bag making machinists.
Oh brilliant, thank you for the starting point! I think Wundaweb looks not quite perf ct as it doesn't stretch, but the same company make similar products that do stretch, such as https://www.vlieseline.com/Products/Tapes/Seam%20Tape%20flexible
completely on the right track here - it's a hot-melt film that's come unstuck, just like a glue-stick in a gluegun. However, there's a wide range of types of film, defined by melting point, thickness, and stretch/non-stretch, so first see if you can re-apply the existing film - I suspect this is a stretch film for jersey, so slightly tricky to get hold of from domestic suppliers.
Make sure you have a solid surface to press on to; a lot of ironing boards are a lightweight mesh, meaning you don't get a consistent connection for stuff like this. Place something like a wood offcut under the ironing board cover/foam base, to give you a solid platform to iron on.
On the inside of the garment, lay a slightly-damp cotton tea towel laid over the top - although polyester has a relatively high melt-point, it's good practice to avoid direct heat damaging the fibres/discolouring. Press and increase the temp until you can tell the film has melted/rebonded. When you've got to this melt point, give it a really solid, 10 sec press - or more if needed.
Even if they've not come apart, it's probably worth running over other similar seams while you're at it.
....and if this fails, yes, Bondaweb - or similar - will do the trick, but won't give the same amount of stretch. I often double, or triple up the layers if I need a more solid-finish. Dead easy to use, but reccommend some baking paper between the iron and the garment; this stops any stray edge glue contaminating the iron, and staining other items down the line. ...and if you wanted to be 100%, stitch along the edge of the folded hem.
@hazzelfrazzel where abouts are you? If you're SE, I can drop some film to you, no problem
You're an absolute legend, as ever! I will give re-application a try over the weekend, using the method you've explained. If not, yeah, I'll try the Bondaweb.
That is a very kind offer, but moved out of London a few years back, so while I used to be very close indeed to your studio in Deptford, I'm now a couple of hundred miles away.
Ah wow, that worked like magic! So much easier than expected! A huge thanks, @Chopsicle
Not sure this is the right sub-forum, definitely more on the"fixin" side of things...
I have a couple of rapha long sleeve pro team jerseys that are very old and I've had great use out of that fit me very well. Much better than the latest pro team jersey I bought from them last summer. They're in pretty good condition apart from some of the seams at the front of the jersey are starting to come undone. I know that rapha do a repair service, but one of these is probably about 7 years old, so I feel that asking them to repair for free would be taking the Mickey. However, I would like to prolong the life of them if possible.
I'm struggling to know what this type of seam is even called to Google repair techniques and materials. They're folded over and then bonded somehow, and it's the bonding that is coming undone. I had assumed it was called a bonded seam, but nope, it seems that's a very different thing on a rain proof garment.
Anyone able to offer any advice on how I could get this fixed, or fix myself? Potentially a question for someone like @Chopsicle?