Does GDPR Apply even if i only have 1 property?
Ideally all documentation is required, however as a bare minumum I would suggest a Privacy Statement.
Do I need to register with the ICO?
Yes, regardless of how many properties you rent you must register with the ICO.
Should my estate agent be doing this?
If you have an estate agent managing your property both you and they need to complete this and both need to register with the ICO.