I know little about them. From what I’ve ready on here much rarer than other Roberts models but maybe so rare it’s a bit niche. Offer a nice Dogsbolx or similar in the right size and I think you’d have several people wanting to bid on it.
Have you had a search on here already?
Last thread about selling one I can find is here
https://www.retrobike.co.uk/threads/roberts-stratos.415792/
The asking price in 2020 I think with a disc drive wheel was £1250 on eBay. As you can see in the thread other prices around £800 were suggested not sure what it sold for. Some of this may be because folk didn’t add value for a disc drive wheel as it doesn’t appeal to them if that makes sense.
I’d guess that other than frame/forks/stem/bars being original, the originality of components on a bike like this one isn’t as important to a buyer as they would have been custom builds and most people on here would do their own build on it. That said if it were a famous owner/rider/show build that would score the points/add the value for originality.
There’s a lot of members with lots of knowledge about Roberts who will hopefully chip in.