Amendments to Tenancy Agreement

Have you considered that you might be stealing their Intellectual Property? You didn't purchase the Agreement from the Agent. You asked them to prepare one for you... it was still theirs I believe. I doubt this will bite you in the real world... but I would suggest you start from scratch... either with an Agreement you've written yourself, an Agreement you've purchased or the free Government model Tenancy Agreement - then you know you're in the clear.

When you finally get there... I would not be writing separate addendums. I would have a single document. And... please realise that your current Tenants are under no obligation to sign a new Agreement while the Tenancy is in operation, so hopefully you have a good relationship with them... especially if you are thinking of introducing new terms and conditions (one would expect these to be restrictions vs. something like an appreciated rent reduction?) to an already-existing arrangement.