Update.
So I decided to just get a new switch, fit it myself, and enjoy driving with working brake lights a.s.a.p! I bought a QH switch like in the link I posted above, it arrived promptly the next day, and armed with my new hose clamp tool I got to work! Gung ho, and confident of my wrenching abilities!
As instructed per this thread, I clamped off the fluid reservoir-to-master cylinder flexible hose, so as not to lose any fluid or allow air into the hydraulic system. I removed the old defunct brake switch and promptly put the new part in, carefully, at first just by hand. Except it simply wouldn’t completely tighten up. At that last turn of the wrench, it felt as if it kept ‘cross threading’. Sure enough, although the brake pedal still felt firm, one push on the pedal and it went straight to the floor, squirting out brake fluid from right where the new switch was fitted. Damn and blast!
So after clearing that up promptly, I swapped over the brass washer again and re-fitted the original albeit defunct switch, and got it tightened up securely, with no fluid leakage. Thankfully I can still drive my car while I try and resolve this issue, although the brake lights still don’t work, obviously.
After that, I closely examined the replacement switch, and, well, I don't know. It looks to me as if there is a defect on the thread, one thread wind down from the tip. Manufacturing fault, or me having been "cack handed"? Whatever, it's dud, it won't seal safely.
Here's the weird thing; there were some remnants of what I suppose was PTFE tape on the thread of the dud switch when I took it out. If some of it stayed in the union, would that really have been able to mash up the thread on the switch!?!? I can't believe Chinesium is so soft that PTFE tape could deform it.
Unfortunately it is very difficult to see in a photograph, although I'm attaching one anyway. What do you guys reckon? Could this be a faulty part, or is it more likely I managed to damage the thread myself?