They still work on maximum weight. Below is a copy of my chat transcript with flythomascook.
.Chat Transcript
10:30:02 AM [Brian] Hi, my name is Brian and I will be assisting you today.
10:31:03 AM [Margaret] Hi Brian, I have purchased 20 kg of hold luggage for my flight. Does this have to go into one suitcase or can it be split into two smaller ones.
10:32:48 AM [Brian] Hello Margaret. You can split the luggage into several cases, however the total amount should not exceed 20kgs.
10:33:05 AM [Margaret] Thanks for that Brian.
10:33:23 AM [Brian] Is there anything else I can assist you with today?
10:33:39 AM [Margaret] No that is all. Many thanks
Ray1951, you can purchase extra weight but it much more expensive than you quoted.
http://www.seatguru.com/airlines/Thomas_Cook_Airlines/baggage.php