Thanks
Who is the registration contact on the domain? If it is you and you have a e-mail address there for you then you should be able to transfer it yourself without the other company knowing about it.
If there's no contractual reason for them to charge you, there's no contractual reason for them not to release the tag and failure to do so could be in breach of their Nominet membership terms, which could in turn result in their membership being suspended.
Nominet's [nominet.org.uk] site contains some useful material about host's obligations and how to make changes [nominet.org.uk] to your domain name which may be of use.
R.