Joint Tenancy Accusation

I recently rented a flat on a 6 months short term lease. It was done through an estate agent acting as agent for the landlord, it was a standard lease.
This is my first rental and as such I payed the whole 6 months rent up front along with a 2 month deposit.
I had only been in the flat a couple of weeks when the landlady accused me living there jointly with my girlfriend, she stays regularly although she has her own place.
As such the landlady says i need to change the lease to a joint lease, she is apparantly worried that if i split from my girl friend and left the flat my girlfriend would have tenancy rights to the property.
I am in UK.
Your opinion please

[OHlandlord]

Your LL is absolutely correct. Most leases only allow guests to stay overnight for a few days for the length of the entire contract. Your GF has been staying there REGULARLY. How often is regularly? On a 6 month lease, she should be staying only one or two nights per month. Is your LL paying for any utilities? If so, she is paying for your GF to stay there. If your lease does not address this, you can be sure it will at renewal time (if she renews your lease at all). She can also take you to court for violating the lease if you signed it to have just one adult living there. A judge may look at you having her stay there regularly as equivalent to living there.

Your LL's fears are justified. Your GF would have tenant's rights if she is staying there regularly. You could move out and the LL could be stuck with an unauthorized occupant that she has no lease with. Either put your GF on the lease or have her go home to her flat.

As LLs, we address this issue quite frequently. If the person is not on the lease, they should not be staying at your unit. If they are a "guest", there is a limit to the amount of time they can stay at your unit. I suggest you start staying part of the time at her flat to lessen the amount of days she stays at yours.