Many thanks for your question.
The extract from the lease appears to make it the ground floor tenant's responsibility to repair the flooring.
As to whether you are partly responsible for remedying the damp, it would depend on the terms of the landlord's repairing obligation. I would need to see the terms of the repairing covenant in order to comment further.
If you can scan and attach to a reply I can take a look for you.
Clause 5(III) page 6 contains the landlord's covenant to repair. This requires the landlord to keep the exterior in repair. I assume that the ground floor lease contains a similar provision in respect of the external parts of the ground floor maisonette.
As you are also the landlord as you own the freehold jointly with the ground floor tenant you, in you capacity as joint landlord, are liable to pay half of the cost of any works to the external parts with the joint landlord (tenant of ground floor).
The ground floor tenant will be liable to pay the costs of the remedial works to the internal parts of the ground floor maisonette.
I hope this helps.
Whilst the above is not to be taken as legal advice I have explained to you how the clauses work which you can in turn explain to the joint landlord.
Please can you accept/rate my reply so that I receive credit for my assistance.