Well, they arrived - and I have to take off my hat to them. Arrived long before the due date.
Inked them up and they all write excellently, out of the box (although there was no box. The ebay listing showed them in the Hero Doctor blister pack, but the pens arrived simply wrapped in bubble wrap - maybe they're counterfeits?)
Nibs are F - EF. I like broader nibs, but am enjoying playing with the F/EF nibs. A useful addition to my B & BB stable.
However, I took one to the grind. Used a coarse grit to get it into shape, then a fine grit (to remove the coarse scratches) then micromesh to smooth it and - hey presto, got a medium/broad with a nice wet ink flow.
I am SO impressed. Thes pens were under £1.20p each, including shipping from China to the UK.
Despite the similar looks to the Parker 51, the comparison stops there. They are slightly thinner and made of plastic, compared to the heavier lucite of the P51 - and it's not just the weight, it feels different to the touch. The nibs aren't gold and the workmanship is, as one would expect, inferior. I won't go on. It's not really fair to compare the two (at their respective prices) - but it's hard to resist, the pens being so similar!
All in all, I am completely satisfied. I'm putting them into the daily rotation, so we'll see how they stand the test of time. I'll include the ebay link, just for reference. No affiliation, just a satisfied customer - despite the pens not being in their original packaging, as the listing promised.