New lambda now getting fault code P0300

Hi folks,

Short version....I don't know what I'm doing!

Long version....my girlfriends 2001 Peugeot 106 1.1 failed it's Mot on emissions with a new lambda given as the fix and the airbag light was on (have sorted the connection under the front seat) The exhaust was an advisory but was so rusted up I decided to change the lot as all three stages were falling to bits. The cat is in the front section of the exhaust and I replaced the post cat lambda with this one

http://www.eurocarparts.com/mobile/ecp/c/Peugeot_206_1.1_2001/p/car-parts/car-body-parts-and-car-exhaust/exhausts/lambda-sensor/?710545711&1&efcd803cf6433455d662c1a0858f892973bb0e10&000210

Since then the management light has been on and it's spluttering when cold. The fault code is P0300. I've checked the lambda has been reconnected properly and am now totally stuck as to how I've messed this up. Any help would be greatly appreciated