‘However they we cheap Silverline ones from eBay”
So, not very surprising. I reckon those are cheaper rubbish than Rolson used to be 20 years ago! Use once and throw away, unless it is simply threading in and out without any serious tightening or loosening. In fact, likely not get to using them once on really tight fixings – they can easily either strip, due to improper fit, or bend.
I bought a Deltec branded set from HIS, last time we were in Inverness. Not used much and I don't try to undo any tight cap screws with a ball end. I use my Stanley or CK for the tough jobs, so they have not been unduly stressed.
The Deltec ones were cheap as there are both hex and torx in the one box. Just a useful set hanging on the wall. Bought because I was impressed with a set bought from HIS and used for motor repairs, by my step SiL.. His were red, my set are yellow, so I am not yet sure if they are comparable on quality.