so here's the thing, you had some shifters on a bike, that bike was bought from a shop, this means the parts were genuine, they lasted well for several reasons, first is that shimano generally is well designed, the materials are generally good, and on the cheaper stuff it tends to be more solid as it's not trying to be lightweight, the most common issue i get with those style of shifters is they can seize up with lack of use, usually a load of lube sorts this out.
now you said you got those shifters for £14, this tells me the ones you have bought are likely fake, as has been suggested. the reason i think this is that the retail price is around £40, at £40 they come in nice shimano box, with both gear and brake cables, both inner and outer. even if these are knocked out cheap by a mail order company they wouldn't be doing them at 65% off as that is way below trade price, this screams fake parts, OR IF WE ARE BEING POSITIVE maybe all the cables were not supplied, or if you were lucky they are genuine but they have been pulled off a bike for some reason and then sold cheap.
if you bought them from Wish, Amazon, eBay or Ali baba or any other similar dodgy website then it is likely you have bought dodgy knock off shifters.
here is a similar set that i had come through my shop, 8 speed, brake lever snapped, gear assembly seized up, generally poor performance and i rarely see rusty stains leaking out of a genuine shifter, and there are small differences in the shape of barrel adjusters, lever blade shape etc. the fake set is the bottom set.
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also, you basically said in one of your posts that essentially your shifters and XTR did the same thing, well yes, but that suggests you don't value better quality components. that seems odd to me, i have a nice vehicle, i value that it's in good condition and quite a high spec, to go back to a more basic spec or older vehicle does not appeal.
