This actually seems fair unless you can absolutely prove the stains were present before you moved in ie they are on the incoming inventory.
I say this because I took my landlord to the small claims court for a similar type of minor damage situation. Briefly:
Incoming inventory showed a door with two marks.
Outgoing inventory showed one mark and one damage (a small circular indentation on the veneer).
I was charged just under £100 for a replacement door.
I knew I had not caused any such damage and felt the damage was so minor that with a cursory look, it could be seen as a mark.
I lost on the balance of probability. Why?
Two points:
First only the outgoing inventory identified damage present and a picture was produced (and yes I re-examined the door after moving out, you are entitled to that). The point on this was that one of the marks on the incoming inventory could have been cleaned off and a mark is a mark, not damage.
More importantly, I expected the proceedings to be myself vs landlord, I did not expect to have 2 extra defendants. Who were they? The landlords husband and the estate agent acting for them. So the Judge has three witnesses (under oath) to say the damage was not there at incoming inventory, against myself. I just saw two marks, in and out.
The Judge was very reasonable and explained his reasoning clearly and yes he applied the law. He even commented about it seeming to be an "ass" at times.
The small claims court is best for this (but you must try to resolve it first with the Landlord or Agent - do all this in writing, keep the letters factual, avoid emotional statements), and I wouldn't say absolutely don't try it, just be prepared for the un-expected. I found the exercise interesting and would go through it again. Be prepared to spend some time getting forms filled in correctly, and be prepared for a bit of a wait (months). You'll probably need to allocate a day for Court proceedings. Incidently, the hearing is usually informal and not in a large court room for these simple disputes. Also you need to be aware that the defendant has the option of using a court near where he lives - it may mean you have the expense of travelling to court.
Here is a starting point:
Making a Claim
Hope that helps.