I just had some outstanding service from Metrology and Quality Services – **LINK**
I bought a few bits from them almost a year ago, including a set of adjustable parallels. I had used a couple but not all of them until this week, the biggest one could not be locked as the fit was a bit sloppy.
Emailed them at approx 5pm tuesday, got a reply at 7.47am the next morning (!!!) picked up by courier the same day, received working replacement this morning.
No affiliation, but very happy – I wasn't really expecting a particularly positive response but couldn't have asked for more than this.