Is it so that they can sent the Land Registry the relevant forms to take their mortgage off your title register?
If so then you just need the leasehold title register if that is the only title your mortgage is secured against (most likely as you only have a share of the freehold).
Ask them if they send the notification that your mortgage has been discharged by END (they send it electronically to the Land Registry) or DS1 which is a form they will either send to the Land Registry themselves or send to you to send to the Land Registry.
A list of lenders that do this is here (and further info about discharging charges)...
http://www1.landregistry.gov.uk/info/articles/ ends/
If they send you a DS1 you can send it to the Land Registry with Form AP1 who will take the motrgage off the title (no charge).
If you ring the Land Registry mentioned on your title register they will be able to talk you through it
Contact details here...
http://www1.landregistry.gov.uk/regional/offic es/
AP1 downloadable here....
http://www1.landregistry.gov.uk/publications/? pubtype=1 (DS1 also on there if you want to see what one looks like).