If the debt had been incurred before you went bankrupt, then (unless it is one of a small category of debts which cannot be proved in bankruptcy - which is unlikely as Mackenziehall deal in ordinary commercial debts) it was included in your bankruptcy and you do not now owe it. This applies even if you did not list the debt on your bankruptcy forms.
However, if you incurred the debt after becoming bankrupt (this seems highly unlikely as it would mean you had obtained �1800 of credit while bankrupt & would therefore almost certainly have broken one of the bankruptcy rules)then you still owe it but it is probably statute barred (i.e. 6 years with no contact, no payment and no acknowledgement of the debt).
I hope you still have your bankruptcy paperwork. As jen suggests, get the OR or IP to confirm the situation and then send these people (recorded delivery & keep a copy) a letter telling them the debt was included in your bankcuptcy and enclosing a copy of your bankuptcy order & discharge certificate.
They are an outfit that can be pretty aggressive and objectionable to deal with - if you can, keep everything in writing, don't phone them and don't let them know your phone number.