TIlly - get your money back but still make it clear that you will take this further because they cannot cancel by giving eight weeks notice for no valid reason. There ARE reasons to which allows them to cancel (force majeure, too few participants) without compensation. Just making a mistake is not a valid reason to cancel, even if they claim it is.
And there are reasons to increase the price - currency/fuel surcharge. Again, this is not what this is about.
But for F to say that one can make a contract and without any valid reason only one side can cancel it is not correct. I'm not a lawyer but i know more than the F staff including the director to whom she refers ( here I must admit that about I was a director of that company ,albeit a junior one and only for a couple of years. Prior and after that I was a travel agent - and also one of F's biggest bookers. )
In fairness to the staff ,even the senior staff actually do believe that they can form a contract, make a mistake purely through their own negligence, wait nine months before realising that mistake and then tell you to shove off, regardless of the extra cost and distress to you.
Don't let it consume you. Write that letter, apply to small claims court (almost free)
and let the matter rest till small claims people contact F, get their reply to which you respond. Takes time. But far far cheaper than a lawyer. Mind you, maybe for a few quid you may want better advice than mine.
On another note - Yes, as posters suggest, you can book an apt and fly out and -maybe for a similar cost.That's not what you were looking forward to for so long. You had booked a top class location with many many facilities included.