If you have booked with Thomas Cook, but are travelling with Thomson, then its Thomson's rules that apply.
So you get 10kgs allowance (thus reducing your hold allowance by 5kgs). As far as dimensions go, if its a sports bag that can be compacted (Isn't stuffed full) then you shouldnt have a problem.
If you have a laptop, put it in the sports bag.
Thomas cook are strictly 5kgs and don't allow you to combine your hold and hand luggage allowance like Thomson do. All airlines have different policies.