I’m 15stone as well, ride a hardtail (like every double hard barsteward on here), you have a nice bike there. Why not just ride it more? Improve your power/weight ratio in the gym? Ahem, or have a big poo? 😆
As said before it will cost you around £1k to loose 2-3 lbs. Why not spend that money on a two week trip to the alps and ride at altitude? I’m sure you wont be complaining when you get back for your first group ride in the spring!