The £300 starting price is very optimistic indeed! The last one I saw on Ebay went unsold with a starting price of £77.
You might be surprised at how many different versions have been sold on Ebay over the past 9 years and I suspect only a tiny few will be the actual original promotional ones. The rest appear be genuine vintage mirrors from the mid 70's but I suspect someone somewhere was manufacturing them when pub mirrors were all the rage. I think pub mirrors were perhaps going out by the 1980's so not sure why there would be copies made then. Other fans who I have had conversations with about this topic have either bought theirs on Ebay or have owned them since new having purchased around the 1975-1977 period.
I started to keep a copy of the Ebay photographs and prices when I was looking to purchase a mirror myself after being outbid several times (a few years back when they were going for up to £350 at times!) I noticed that there were subtle differences in some of the mirrors being sold so studied them a bit more carefully and decided not to bid on ones which looked a bit "off". They may well date from the seventies and so would be genuine vintage mirrors but perhaps not the actual original promotional mirrors. I think many of the ones that turn up now have been reframed at some point so hard to compare.
Aspell Saggers & Co also made the small glass FYP ashtrays.