I've read through this, & I'm still confused.
1. Are you the only owner of the property? If not (& any joint owner will not necessarily be on your mortgage), could it be that the other owner has a debt & the interest which has been registered relates to them?
2. If the above is not the case - in other words, if you are the sole owner - you can get your title deeds from landregistry online for a few £. Do so & study them to see what you can find out. That, together with the reply from the Land Registry when you get it, should make the situation clearer to you.
3. You will then probably have to contact the solicitor who arranged for the charge to be put on, in order to find out why they did it, who instructed them (the name of the person or company should, in any case, be on the title deeds) & how they propose to get it removed.
4. Do not leave this - the longer it stays unresolved the more difficult it will be to sort it out.