To put this in layman's terms, in 2005 the then Labour government (mindful that the prior film tax-relief was being abused by tax-avoiders) set up a new scheme that was intended to promote film production in the UK while making it harder for companies to simply dump their money into loss-making films.
The overall plan for the new scheme was that for each pound spent investing in a film, the investor and the filmmaker (on each side of the "partnership") would receive a comparable amount of tax relief against their overall tax bill with a greater amount being recouped if the film was completed and an even larger amount being recouped if the film made a profit. Unfortunately, shortly afterwards numerous trading companies were set up (mainly by large legal firms) to channel money from wealthy individuals into film production, mainly for the purposes of gaining the tax relief.
Typically the scheme would identify and purchase the rights to an upcoming Hollywood blockbuster and lease those same rights back to the film production company, who were in the process of producing the film, with the lease agreement generating a revenue stream for the partnership. Films that were part of large ongoing franchises (and hence had an assured profit-margin) were especially attractive.
Over the next 10 years, a number of these schemes were unwound. The key to the Eclipse 35 ruling is that a company that was set up for the purposes of finding investors ("partners") for films is acceptable. A company that has been set up for the purposes of evading taxes is not. The latest ruling is that the business has to be 'trading' or else it's simply a tax-avoidance vehicle.
There's some additional reading here, here and here that may prove useful.