Jan - It is a rentcharge that they are enforcing. I'm not a lawyer by any means but I'm having my own battles with another parasite which has educated me somewhat in this issue.
What they are charging you is based an an old law, (section 121, law of property act, 1925). Technically, if you haven't paid their demand within forty days of asking, they can take possession of your property until the charges and costs are paid. Of course, I don't know anyone who has actually done this, but a county court judgement is certainly possible.
Rather than contact the company by phone, continue all correspondence with them via email so that you have a record of everything. Let them know that you have already sent the payment and it's unfortunate that they did not receive it. As much as you cannot prove that you sent the cheque, they cannot prove that they didn't receive it. I'm sure the County Court would not look on a company that did not take any reasonable steps to resolve the matter before going to court.
Have they given you any other method of payment apart from cheque? If not, ask them to give you an option to pay online so that there's a record of payment. I personally would be loathed to pay the admin fee if i'd sent the cheque, but you might want to do so if you just want the problem to go away. Alternatively, call their bluff and be prepared to go to court to contest it if they insist on the costs.
Finally, obtain an application for a Redemption Certificate from the Rentcharges Unit, Department for Communities & Local Government, 3rd Floor, 12 Princes Parade, Princes Dock, Liverpool L3 1DE. ( Tel: 0303 4444558) to redeem the rentcharge and get rid of your problem once and for all. It will cost you about 16 times the original cost, (less than £200), but it's better than the hassle of having to deal with parasites.
Read further here:
www.cluttoncox.co.uk/site/blog/conveyancingblog/how_to_redeem_rentcharge_in_bristol.html