Thanks for the quick reply redfred. It made me question if I had imagined it but on checking the Tomas Cook web site it states:
If you wish to travel with your own equipment on a charter flight there is a charge of £13 made for each pair of skis or snowboard. This carriage can be pre-booked up to 14 days before departure, after this you will have to book ski carriage at the Thomas Cook Airline desk at your departure airport on your day of departure at a cost of £25 per item.
It is clearly only an issue if you travel on charter flights.
I'd still like to know what its for and where it goes though!