If you have bailiffs round, then it will have been to county court. If it has been to county court then you should have had letters delivered to you.
Either your flat mate has been stealing your mail, or (for some reason) they have not been served on you.
Very important: get a copy of your credit file from www.experian.co.uk and www.equifax.co.uk
See if a CCJ has been registered against you and also if an "association" has been made between you and your flat mate - if so (and on the assumtion that you are not financially linked), file a "notice of disassociation" - if you don't, your credit rating may be harmed.
If you do have a CCJ and your flat mate has been stealing your mail, you may need to go to court and ask for the judgement to be "set aside". You will then need to pay the amount outstanding that you are liable for - (you are liable regardless of if you have paid your flat mate). You can take your flat mate to the county courts to try and get a judgement against him - will cost you around �30 but may not be much point!