The law is remarkably vague about the border between being inspired by something and copying it. In your case, if you are doing this for pleasure then there's no problem with that. But if you start selling your art, a) it's more likely that the owner of the original image will notice the similarity, and b) it starts to make it worthwhile trying to extract some money from you!
The most important UK case involving similar facts is Bauman v Fussell
from 1978, in which it was found that the artist had been inspired by a photograph of two fighting cocks, but his resulting painting produced an entirely different aesthetic and did not infringe the earlier work. However that case was based on the 1956 Copyright Act and occurred at a time when the EU (or EEC as it was then) had not established a coherent legal framework for copyright. Thus today we need to view the issue from a European jurisprudential point of view. Sadly no clarity emerges.
Here are two*
contrasting cases from Europe where technically the same approach to deciding on copyright infringement is supposed to operate: a Swedish Supreme Court decision
, and a decision of the Belgian courts
. The Swedish case bears some similarities to your example, in that the painting contains an element of fantasy which was not present in the photograph. It is worth noting that in the second case, part of the defence was that the painting was satirical in its intention, although this doesn't seem to have swayed the court.
From your description, it sounds as if your final version is sufficiently far from the original that it could rely on the decision in Bauman v Fussell
to avoid a claim of infringement, but you should make every effort to incorporate as little as possible from the original work. You mention the phrase fair use, although that is a doctrine from US copyright law (where it is highly likely that your work would qualify as fair use, due to the transformative nature of the second work). However, under the UK doctrine of fair dealing, aside from the pastiche/parody exception I mentioned in my earlier reply about Punch and Judy, there is no help there for what you propose, assuming that the painting is going to be made available to the public (ie it isn't just for private research or study).
. We now have a third decision
, this time from a French court which offers a slightly different analysis of the issue.