Not good at all!!!
I see your solicitors were supposed to liaise about the charge. This info is important, have you anything in writing?
I'd give the SRA (our regulatory body) a ring and talk it through with them...
http://www.legalcomplaints.org.uk/home.page
They often ask for an approach to be made to the solicitors by you first, if so then put it in writing and send it FAO the Senior Partner.
In my opinion, this is something the SRA need to know about if the firm is practising in this way. Make sure you tell them about your independant legal advise about being poorly informed esp if there was a potential conflict of interest!
I'd definitely check if you need to pay it. I'd say it should be paid by the solicitors to get rid of the charge and, if you are liable to pay it then agree a reasonable (and I'd hope interest and fee free) repayment structure with them.
Info on the firm should be found here...
http://www.lawsociety.org.uk/choosingandusing/ findasolicitor.law
I hope this helps and you get things sorted :)