Do you have leasehold title registered in your name? Most likely a printout of the property register from the Land Registry showing your title number, details, you as owner etc...
Your leasehold title will have it's own disctinct leasehold title number if it is registered with the Land Registry. The freehold has it's own disctinct title number(s).
Your title is the lease and anything else affecting your interest in the land (eg charges such as mortgages). The lease is the main ownership document setting out the length, rent, terms etc... of the ownership.
The titles layer on each other ie freehold as ultimate title, your leasehold as a title as a layer above that....
When did you purchase the lease (in case before compulsory registration)? Can you ever remember seeing a copy of the lease eg when you purchased or there were any other circumstances in which you acquired the property which could account for this eg left it under a will or purchase it at an auction or similar with limited details available or didn't use a legal advisor?
The Land Registry would, these days, copy the Lease and store it electronically if the property is registered. If it is registered I would be surprised if you have absolute title as, if the LR has never seen the Lease, I wouldn't have thought they would have granted that, more likely good leasehold title so, if you have title absolute then it would suggest a copy of the lease has been seen by the LR somewhere along the line.
Also bear in mind that leases can be split, the original lease could have covered a much larger area than just your property, especially as it's an old one. It may be that the original leasehold was sold off in parts and now consists of a number of separate leasehold titles (if that makes sense) so, if this is the case, it may be more likely that there is a copy obtainable somewhere in someone's title/deeds/records.