I'm not sure exactly what that band is for. It could be there to reduce
radiation of the transformer's eddy currents and thus satisfy emission
regulations. It could be there as part of the heat transfer mechanism.

The band doesn't look like it was soldered or a continuous strip that I
ripped apart (it's not *that* fragile, nor did I "gorilla" it *that* much
when separating the heat sink). I'll lay it down to the lams when I replace
the adhesive.

For "glue", I would use silicone adhesive/sealant eg maplin N71BZ. It
may not be what they used originally, but it should do the trick nicely.

So you don't think the glue is thermally conductive at all? Silicon would,
surely, insulate.

For the interface between heatsink and copper - I would try to get as
much mechanical metal to metal contact as possible. Then put a very,
very thin coating of heat sink compound on. But the aim would be to
ensure that the heatsink remains in close contact with the heat source -
even if that means adding additional clamping.

Other components won't allow such close contact between transformer and heat
sink. Some heat sinks soldered to the PCB make contact with this plate above
the PCB which leaves ~5mm gap between lams and the plate. I'd like to make
contact between the 2, but this isn't easily do-able.

If the copper pad looked to be providing electrical, as well as
mechanical contact, I'd want to make sure that was re-established too.

As best I can tell, it isn't an electrical issue; there isn't a soldered
connection nor a solid physical one. The two ends of the band just seem to
meet at the corner without any assurance that they'd be connected. (Upon
closer examination there's what remains of a drop of... cyanoacrylate? ...
under the end of the copper band. I'll re-do that so it's laying back down.)

Still not sure about the glue and whether it may have been thermally
conductive...