Curious really, just want to know...

Are there any restrictions on the topic/subject/plot of an adventure, does anyone know. I am thinking more on the lines of fan fiction types. eg. Basing the adventure on a book and/or film and/or T.V. series and/or radio series (after I've assembled my
Sub-Etha Sens-O-Matic), using the names of existing characters (Sherlock Holmes or a dwarf called 'Gloin').

Ummm...

To save people a lot of time =)

https://textadventures.co.uk/forum/design/topic/5797/copyright

This has been brought up before and certainly people can sound off on this thread, but the link above is a lengthy conversation about copyright infringement, authoring expectations, etc.

Good luck!

Is it alright to link to images? like URL's? although probably never going to put my adventure on the public view

Reading through the article XanMag links to, I think it depends on the image in question, the subject of the image. A photo of a tin of baked beans would be copyrighted because of the manufacturer of the tin, but a plate of beans on toast wont be. Equally, if there are people in the photo, their likeness is their copyright, so you'd have to get their permission to use their likeness in your game. Even some clipart images may be subject to copyright.

Theft/vandalism/plagerism of (someone else's) property (ie: fan fiction) is illegal/criminal, (as a person's property is vital and very important, which must be protected for having a functional society). Even if you create something that is otherwise 99-100% original/unique, if it's (found to be in court/lawyers/law) directly/strongly based upon someone else's property, then that is still theft/vandalism/plagerism of someone's property and thus illegal/criminal. Though, confusingly if you create something that is strongly based on your own idea, you can use someone else's property if you modify it enough that it's your creation, separate from their creation, then you may do so (albiet you still got to win/prove it in court... that you are indeed doing so, and thus have the legal right to do so).

Basically, in terms of legality, all creativity is subject to being illegal/criminal (theft/vandalism/plagerism of someone else's property), EXCEPT for stuff that is legally declared as being free/public/commons and/or you got documentation/proof of the owner giving you the right to use/modify his/her property and/or the the ownership-duration has ended (thanks to Walt Disney / Disney company), copyright lasts ~100 years: http://copyright.nova.edu/mickey-public-domain/ , Disney hates competition! Disney stole the Grimm Tales as they never got their work protected, using it for their own famous/classical disney works, and now they keep their own property from others being able to use it. Bunch of corrupt hypocrit and greedy corporate bastards).

http://copyright.lib.harvard.edu/states/ (click on state for copyright resources)
https://cyber.harvard.edu/metaschool/fisher/domain/tm.htm

http://fairuse.stanford.edu/overview/introduction/intellectual-property-laws/ (click on the next page buttons, there's lots of pages, some of those pages are directly below)
http://fairuse.stanford.edu/overview/website-permissions/websites/
http://fairuse.stanford.edu/overview/website-permissions/transferring-information/
http://fairuse.stanford.edu/overview/website-permissions/linking/
(etc etc etc pages)

https://www.uspto.gov/intellectual-property-ip-policy (click-on/look-at trademarks and patents links/buttons/resources too)
https://www.uspto.gov/learning-and-resources/ip-policy/copyright/copyright-basics
https://www.uspto.gov/trademark/laws-regulations

https://www.sba.gov/starting-business/learn-about-business-laws/intellectual-property-law
https://memory.loc.gov/ammem/awhhtml/awlaw3/property.html

some cases:

http://www.ipwatchdog.com/2017/03/25/appeal-copyright-led-zeppelin-stairway-to-heaven/id=79696/
http://digitalcommons.law.scu.edu/facpubs/79/

Bunch of corrupt hypocrit and greedy corporate bastards

That sounds about right. You forgot to mention they are litigation-happy fatcats : ) Oops, just violated a Disney anti-defamation provision somewhere...

the 'merchants of cool' science/educational/documentary sociology video briefly mentions/shows disney as one of the companies (as one of the banners in the very beginning intro - well actually this was a decade ago, looks like they removed this beginning intro from it) having power/control over/of society, but they don't dare discuss anything about disney in the entire video

ht.tp://www.pbs.org/wgbh/pages/frontline/shows/cool/view/
ht.tps://www.youtube.com/watch?v=9elU-EOEUXE