Even at £28+vat on offer I think I would be looking elsewhere for a cheap TiN coated or cobalt set, I have Presto and Addax drill sets, TiN coated and Cobalt, they are good for the money, although my sets only go up to 10mm.
The Hertel set look to be plain HSS and probably made in China, as most are. Likely no better than a £20 lidl set. Plain HSS can struggle on stainless, I prefer m35 or m42 cobalt bits.
Cobalt drills are ok in a pillar drill but not so good in a hand held drill as they are easy to break, especially in smaller sizes.
The Dormer A002 drills are very good & work well on most materials. I had to drill around 150 holes in a steel pipe & I bought a box of 5 x 3.2 mm A002 bits expecting to use more than 1 bit. One bit drilled all 150 holes & has drilled many more since.
Aye, they used to be made in Peterhead on the N-E coast of Scotland, near to my home town (at least they had a factory there, not sure if the drills were made elsewhere).
I keep a set of the Banggood/Drillpro cobalt bits on the pillar drill, for use only on there or the lathe. They are absolutely fantastic, will chew through any material very rapidly.
I've tried them on a cordless drill & it usually doesn't end well. Unless you react very quickly they can snap if they snag when you break through.
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I thought they would be awful but they are brilliant. Shanks are rather soft which means if you use them in a battery drill they will spin and gouge but in a lathe or milling machine they are fantastic. I have never broken one. The 3 bigger ones have 10mm shanks. I have managed to drill through some wheel bolts/studs no problem.
I always go gently when drilling and use plenty of soluble cutting fluid or rocol.