Rindless,
I have not used Lexmark printers before, epson only, but have bought compatible carts from
www.7dayshop.com
www.mx2.org
www.choicestationary.com
www.inkcarts.co.uk
The epson compatibles are very cheap and Ive never had problems that i didnt have with the originals.... I had a quick look and the compatible lexmarks are pretty expensive but still a bit cheaper and some places will pay you a couple of quid for your empty lexmark carts.
The carts must have the printheads built into them, its the only reason i can think of for the highish price.
Have a look at some of the above addresses.
Vex.