One of the tenants on original lease said he will only pay his share of rent and wont pay for vacant room. The original letting agreement said he was jointly and severally liable for rent.
I am seriously considering giving written notice.
There is one tenancy. The tenants, jointly, have to pay the entire rent. If they fail, the tenancy, as a whole, falls into arrears. If the tenancy is ended, they are all evicted. It doesn't matter which of them actually paid any rent.
That your lease says this outright is good for you, though I think it is implicit even if it doesn't.
Failure to pay the rent is a valid trigger for a NoT (there is a process though).
Tenants who have below market rent should 100% continue to pay their rent, in full. (Well, unless they are planning to leave anyway, and they want their last year or so to be rent free). A low rent is worth protecting by them.
It would be bad for them to give an easier/clear justification for eviction. Additionally, an eviction for failure to pay the rent is not a "no-fault" eviction, so the landlord can raise the rent to market once the eviction completes.
New tenants can only be added to the tenancy with your consent (or by RTB/court order). You have to be consulted, and consent, to upgrade a licensee (paid lodger) to a tenant.
This means that currently there is one tenant (the original one) and a licensee. The original tenant has to pay the entire rent if he wants the tenancy to continue. I am assuming you didn't agree to an upgrade of the other person.
If you agreed to the other person being a tenant, then they are both tenants. That might be implicit if you knew they were there (especially if they have lived there for more than six months).
If someone lives in the house for six months, or more, you can't refuse to make them a tenant if it would be "unreasonable". Up to six months, you can just refuse outright. Either way, they have to ask and you have to formally consent to the upgrade.
Does a new occupant in a house share who moves in after 1st March have new legal rights compared to the existing occupants there before 1st March?
They shouldn't. The tenancy is what is key, they would be joining the original tenancy, under the old rules.
You should get that in writing probably. Probably get them to agree that they are joining an existing tenancy and also agree to the lease.
Also, the RTB (or courts) might rule otherwise, but generally tenancies started before 1st March should stay under the old rules until the tenancy fully ends. The tenancy ends when all tenants of the tenancy leave and the landlord gets vacant possession again.
There is some question about pre-22 tenancies which are 6 year tenancies, but the feeling is that they will upgrade to pre-26 (tenancy of unlimited duration) tenancies and then stay that way.
Key point is to determine who is and is not a tenant.
The original tenants who signed the lease are tenants (if still living there).
(Former) tenants who have left are not tenants (ideally they would have notified you, and you would have notified the RTB)
If you upgraded someone to tenant, then they are tenants (if still living there).
Anyone living there with permission from the tenants is a licensee only.
A licensee who has been there for more than six months might be entitled to be upgraded to a tenant, but that requires that you actually do it.
If you are sending a NoT, you need to send it to
all tenants individually. There is a whole complex procedure.
If might be worth joining the
IPOA as they have guides and support their members with more info than is on the public site. I am not a member but many people have commented that following their advice works better, though it is still a minefield.