Yeah, you really just need to have it welded back on. You don't need to to strip the bike down for that. Ride the bike to a welding shop and they can glue it back together on the spot. That fender doesn't even need to fight off. That's a 5 min job, cleaning and welding. By the time you ride home it'll be perfectly cool to dab some black paint back on
I don't know how long it will last, but a buddy ordered a Chinese Tig to his door for like $200. Of course you've got to do a bottle/regulator on top of that, but a Tig fuse weld would be perfect in this situation.
I don't know what the Harbor Freight equivalent would be to rightsideup in England, or if there is any, but there are inexpensive welders to pick up. The thing with cheap welders + being a beginner is not knowing if the machine is fighting you or if it's you.
Thin stuff welds fine with any flux core machine. Could be a $500 lincoln or an $80 amazon or harbor crap machine. The splatter is more of an issue without gas shielding. The frame to bracket joint is failure prone. Bad design and a good shop might have a very simple fix to eliminate the stress point so it doesnt break again.
The p clamp idea was great...not for looks or original design. The clamp would distribute the load. Fab vs welding!
I think to do it without welding, you'll have to replace it with something different:
My first thought was a T shaped piece, upside down, bolted through the rear hoop, and then hammered over top to reach the knob at the back of the seat.
Now that I've thought a little more, if you don't want to permanently modify the rear hoop by drilling holes as mentioned above, you could likely bend a Z shaped piece that bolts on to the two fender mounting bolts, comes up inside the hoop and then flattens out to reach the knob.